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EDWARD  CAPPS 

STARR  WILLARD  CUTTING  ROLLIN  D.  SALISBURY 

JAMES  ROWLAND  ANGELL      WILLIAM  I.  THOMAS  SHAILER  MATHEWS 

CARL  DARLING  BUCK  FREDERIC  IVES  CARPENTER         OSKAR  BOLZA 

JULIUS   STIEGLITZ  JACQUES  LOEB 


THESE  VOLUMES  ARE  DEDICATED 

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OP  OUR  TIME   AND  COUNTRY  WHO   BY   WISE  AND   GENEROUS  GIVING 

HAVE   ENCOURAGED   THE   SEARCH   AFTER   TRUTH 

IN  ALL   DEPARTMENTS   OP   KNOWLEDGE 


THE  STUDY  OF  STELLAR  EVOLUTION 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 

agents 
THE  BAKER  &  TAYLOR  COMPANY 

NEW    YORK 

THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDON    AND    EDINBURGH 


PLATE  I 


THE  GREAT  NEBULA  IN  Andromeda 
Photographed  with  the  24-inch  reflecting  telescope  of  the  Yerkes  Observatory  (Ritchey) 


THE  STUDY  OF  STELLAR 
EVOLUTION 


AN  ACCOUNT  OF  SOME  RECENT  METHODS 
OF  ASTROPHYSICAL  RESEARCH 


GEORGE  ELLERY  HALE 

/  < 

FORMERLY  OF  THE  DEPARTMENT  OF  ASTRONOMY  AND  ASTROPHYSICS 
NOW  DIRECTOR  OF  THE   MOUNT  WILSON   SOLAR  OBSERVATORY 


THE  DECENNIAL  PUBLICATIONS 
SECOND  SERIES    VOLUME  X 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


Copyright  1908  By 
THE  UNIVEESITY  OF  CHICAGO 


Entered  at  Stationers^  Hall 


Published  May  1908 
Second  Impression  August  1909 


Composed  and  Printed  By 
The  University  of  Chicago  Pre 
Chicago,  Illinois,  U.S.A. 


PREFACE 

As  first  planned,  this  book  was  intended  to  serve  as  a 
handbook  to  the  Yerkes  Observatory.  Many  inquiries 
regarding  the  observatory's  work,  made  by  the  numerous 
visitors  received  there  annually,  seemed  to  call  for  a  printed 
explanation  of  the  purposes  in  view  and  the  observational 
methods  employed.  Removal  to  California  and  new  duties 
connected  with  the  organization  of  the  Mount  Wilson  Solar 
Observatory  caused  a  modification  of  the  project.  I  finally 
adopted  the  plan  of  describing  a  connected  series  of  inves- 
tigations, laying  special  stress  on  the  observational  methods 
employed,  in  the  hope  of*  explaining  clearly  how  the  problem 
of  stellar  evolution  is  studied.  The  advantage  of  using  con- 
crete illustrations  drawn,  in  large  part,  from  personal  experi- 
ence, and  the  desire  that  the  book  should  be  of  special  service 
to  visitors  at  the  Yerkes  and  Mount  Wilson  Observatories, 
are  sufficient  reasons,  I  trust,  for  the  otherwise  undue  pro- 
portion of  space  devoted  to  these  institutions. 

The  omission  of  such  important  subjects  as  the  theories 
of  temporary  and  variable  stars;  Sir  George  Darwin's  dis- 
cussions of  evolution  as  affected  by  tidal  friction;  Vogel's 
and  Pickering's  photometric  and  spectroscopic  studies,  and 
the  researches  of  the  latter  on  the  distribution  of  stars  of 
various  types;  Campbell's  investigations  of  stellar  spectra, 
and,  to  mention  no  other  work,  his  development  of  the  spectro- 
graphic  method  of  determining  radial  velocities,  sufficiently 
indicate  that  I  have  made  no  attempt  to  deal  with  the  general 
problem  of  stellar  evolution,  or  to  offer  anything  approaching 
an  a<  k'n  UH!  <  ieseription  of  the  observational  methods  of  astro- 
physics. The  .various  researches  described  are  chosen  rather 
arbitrarily,  in  some  cases  with  more  regard  for  my  personal 


x  PREFACE 

acquaintance  with  the  facts  than  because  of  their  intrinsic 
importance.  I  trust,  however,  that  although  this  method  of 
treatment  has  necessarily  resulted  in  a  fragmentary  exposi- 
tion of  the  subject,  the  book  will  serve  to  show  how  the 
problem  of  stellar  evolution  is  attacked  along  converging 
lines,  leading  from  solar,  stellar,  and  laboratory  investiga- 
tions. 

I  wish  to  express  my  thanks  to  Sir  William  Huggins ;  to 
Messrs.  Adams,  Ellerman,  Olmsted,  and  Ritchey  of  the 
Mount  Wilson  Solar  Observatory;  to  Professors  Barnard, 
Burnham,  and  Frost  of  the  Yerkes  Observatory;  to  Professor 
Campbell  of  the  Lick  Observatory;  to  Professor  Pickering 
of  Harvard  College  Observatory ;  to  Mr.  Abbot  of  the  Smith- 
sonian Astrophysical  Observatory;  to  Professor  Lord  of  the 
Emerson  McMillin  Observatory;  and  to  Professor  Ames, 
Mr.  Jewell  of  Johns  Hopkins  University,  for  photographs 
which  appear  in  the  plates.  I  am  also  indebted  to  the 
Astrophysical  Journal  and  the  Publications  of  the  Yerkes 
Observatory  for  many  cuts,  and  to  Messrs.  Ticknor  &  Co. 
for  permission  to  reproduce  Langley's  drawing  of  a  typical 
sun-spot.  I  am  under  special  obligations  to  my  colleague, 
Mr.  Ellerman,  to  whom  are  due  most  of  the  photographs 
of  instruments,  buildings,  and  landscapes  which  appear  in 
the  plates,  besides  many  of  the  solar  and  stellar  photographs 
taken  from  our  joint  papers  in  the  Astrophysical  Journal 
and  elsewhere. 

G.  E.  H. 

PASADENA,  CALIFORNIA 
November,  1907 


CONTENTS 

CHAPTER  PAGB 

I.     THE  PROBLEM  OF  STELLAR  EVOLUTION      -  1 
II.     THE  STUDENT  OF  THE  NEW  ASTRONOMY    -               .  -       J£ 

III.  THE  SUN  AS  A  TYPICAL  STAR    •  -       15 

IV.  LARGE  AND  SMALL  TELESCOPES  -      20 
V.     ASTRONOMICAL  PHOTOGRAPHY  WITH  CAMERA  LENSES  -      27 

VI.     DEVELOPMENT  OF  THE  REFLECTING  TELESCOPE  -      38 
VII.     ELEMENTARY  PRINCIPLES  OF  SPECTRUM  ANALYSIS  46 
VIII.     GRATING   SPECTROSCOPES  AND  THE  CHEMICAL  COMPO- 
SITION OF  THE   SUN     -  56 
IX.     PHENOMENA  OF  THE  SUN'S  SURFACE  -  -      67 
X.     THE  SUN'S  SURROUNDINGS  -      73 
XI.     THE  SPECTROHELIOGRAPH   -  -      82 
XII.     THE  YERKES  OBSERVATORY  -      97 

XIII.  ASTRONOMICAL  ADVANTAGES  OF  HIGH  ALTITUDES  111 

XIV.  THE  MOUNT  WILSON  SOLAR  OBSERVATORY  -     121 
XV.     THE  SNOW  TELESCOPE  131 

XVI.     SOME  USES  OF  SPECTROHELIOGRAPH  PLATES       -  -     139 

XVII.     A  STUDY  OF  SUN-SPOTS  151 

XVIII.     STELLAR  TEMPERATURES     -  165 

XIX.     THE  NEBULAR  HYPOTHESIS  -     175_ 

XX.     STELLAR  DEVELOPMENT      -  187 

XXI.     THE  METEORITIC  AND  PLANETESIMAL  HYPOTHESES  204_ 

XXII.     DOES  THE  SOLAR  HEAT  VARY?           -        -        -  -     212 

*  •   ••    — 

XXIII.  THE  CONSTRUCTION  OF   A  LARGE  REFLECTING  TELE- 
SCOPE -    219 

XXIV.  SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS  -    230 
XXV.     OPPORTUNITIES  FOR  AMATEUR  OBSERVERS  -        -  -     243 


INDEX 


251 


CHAPTER  I 
THE  PROBLEM  OF  STELLAR  EVOLUTION 

IT  is  not  too  much  to  say  that  the  attitude  of  scientific 
investigators  toward  research  has  undergone  a  radical  change 
since  the  publication  of  the  Origin  of  Species. .  This  is  true 
not  only  of  biological  research,  but  to  some  degree  in  the 
domain  of  the  physical  sciences.  Investigators  who  were 
formerly  content  to  study  isolated  phenomena,  with  little 
regard  to  their  larger  relationships,  have  been  led  to  take 
a  wider  view.  As  a  consequence,  the  attractive  qualities  of 
scientific  research  have  been  greatly  multiplied.  Many  a 
student,  who  could  see  in  a  museum  only  a  wilderness  of  dry 
bones,  now  finds  each  fragment  of  profound  interest  if  the 
part  it  plays  in  a  general  scheme  of  evolution  can  be  made 
clear.  The  color  and  structure  of  any  animal  or  plant,  the 
minute  modifications  which  distinguish  one  variety  from 
another,  take  on  new  significance  when  considered  as  evi- 
dences of  development.  Their  appeal  to  the  microscopist, 
or  to  anyone  who  finds  delight  in  intricacy  of  structure  or 
beauty  of  form,  is  quite  as  great  as  before.  But  to  the  stu- 
dent whose  interest  is  not  aroused  by  such  details,  perhaps 
from  lack  of  technical  knowledge,  or  from  the  feeling  that 
these  matters  are  trivial  as  compared  with  the  larger  problems 
of  science,  such  minor  peculiarities  must  appear  in  a  new 
light.  Their  true  significance  becomes  apparent,  and  the 
importance  of  studying  them,  once  perhaps  underestimated, 
now  requires  no  demonstration. 

In  astronomy  the  idea  of  evolution  goes  back  to  a  very 
early  period.  In  a  crude  and  grotesque  form  the  traditions 
of  the  earliest  peoples  invariably  struggle  to  account  for  the 

1 


STELLAR  EVOLUTION 


origin  of  the  Earth  and  its  inhabitants.  On  a  much  higher 
plane  stand  the  speculations  of  the  Greek  philosophers  and 
of  those  who  have  followed  them  in  the  centuries  preceding 
our  own  time.  All  schools  of  astronomers,  dealing  in  some 
instances  with  purely  philosophical  and  theoretical  considera- 
tions, and  in  others  basing  their  conclusions  upon  known 
facts  of  observation,  have  sought  in  their  turn  to  explain  the 
origin  of  the  solar  system  and  the  larger  relationships  that 
obtain  in  the  universe  as  a  whole.  In  the  eighteenth  century 
these  speculations  reached  their  climax  in  the  nebular  hy- 
pothesis of  Laplace,  which  still  remains  as  the  most  serious 
attempt  to  exhibit  the  development  of  the  solar  system. 
Attacked  on  many  grounds,  and  showing  signs  of  weakness 
that  seem  to  demand  radical  modification  of  Laplace's  original 
ideas,  it  nevertheless  presents  a  picture  of  the  solar  system 
which  has  served  to  connect  in  a  general  way  a  mass  of  indi- 
vidual phenomena,  and  to  give  significance  to  apparently 
isolated  facts  that  offer  little  of  interest  without  the  illumina- 
tion of  this  governing  principle. 

It  will  be  seen,  therefore,  that  the  idea  of  evolution  and 
development  is  by  no  means  new  to  the  astronomer.  But  it 
may  nevertheless  be  maintained  that  it  has  occupied  a  more 
important  position  since  Darwin  published  his  great  work. 
In  1859,  the  very  year  of  the  publication  of  the  Origin  oj 
Species,  Kirchhoff  first  succeeded  in  determining  the  chemical 
composition  of  the  Sun  by  the  aid  of  the  spectroscope.  His 
fundamental  discovery  marked  the  entrance  of  this  instru- 
ment into  the  field  of  astronomical  research  and  established 
on  a  firm  basis  the  new  science  of  astrophysics.  The  impor- 
tance of  spectroscopic  investigations  in  their  relationship  to 
evolution  was  soon  made  clear.  Within  a  single  decade  the 
study  of  stellar  spectra  by  Huggins,  Rutherfurd,  and  Secchi 
had  shown  that  the  stars  may  be  divided  into  several  classes, 
characterized  by  distinctive  peculiarities  in  their  luminous 


THE  PROBLEM  or  STELLAE  EVOLUTION     3 

emission  and  marking  definite  stages  in  an  orderly  process 
of  development.  Following  close  upon  this  pioneer  work 
came  the  capital  discovery  by  Huggins  of  the  gaseous  nature 
of  the  nebulae,  and  the  relationship  of  these  celestial  clouds 
to  the  stars  which  they  enshroud.  In  these  filmy  masses  of 
luminous  gas  it  appeared  probable  that  the  stars  had  their 
origin,  taking  form  after  long  ages  of  condensation,  through 
processes  regarding  which  our  ideas  are  still  vague  and  ill 
defined.  Belief  in  such  a  mode  of  development  has  been 
greatly  strengthened  through  the  results  of  recent  investiga- 
tions, and  especially  through  the  discovery  by  Keeler  that 
of  120,000  nebulae  strewn  over  the  heavens  fully  one-half  are 
distinctly  spiral  in  form.  This  far-reaching  conclusion, 
coming  at  the  end  of  the  nineteenth  century,  is  furnishing 
materials  for  those  who  seek,  through  modification  of  the 
nebular  hypothesis,  to  provide  a  sound  and  sufficient  explana- 
tion of  the  development  of  suns  like  our  own. 

We  are  now  in  a  position  to  regard  the  study  of  evolution 
as  that  of  a  single  great  problem,  beginning  with  the  origin 
of  the  stars  in  the  nebulae  and  culminating  in  those  difficult 
and  complex  sciences  that  endeavor  to  account,  not  merely 
for  the  phenomena  of  life,  but  for  the  laws  which  control  a 
society  composed  of  human  beings.  Any  such  consideration 
of  all  natural  phenomena  as  elements  in  a  single  problem 
must  begin  with  a  study  of  the  Sun,  the  only  star  lying  near 
enough  the  Earth  to  permit  of  detailed  investigation.  The 
knowledge  thus  derived  may  then  be  applied  in  researches 
on  the  nebulae,  and  in  the  elucidation  of  spectroscopic  obser- 
vations of  those  stars  which  represent  the  early  period  of 
stellar  existence.  According  to  present  views,  the  state  of 
development  attained  by  the  Sun  is  that  of  maturity,  if  not 
of  decline.  After  it  come  the  red  stars,  which  represent  the 
last  stages  of  luminous  stellar  life.  Even  the  extinction  of 
light  due  to  continued  cooling  is  not  sufficient  to  exclude 


STELLAR  EVOLUTION 


altogether  from  the  astrophysicist's  study  those  dying  stars 
which  represent  a  condition  lying  between  that  of  a  glowing 
sun  and  a  dead  planet  like  the  Earth  or  the  Moon.  Through 
one  of  its  many  remarkable  properties,  the  spectroscope 
enables  us  to  detect  the  presence,  and  sometimes  to  deter- 
mine the  dimensions,  of  vast  bodies  which  have  resulted  from 
the  cooling  of  former  suns.  It  will  be  the  object  of  this 
book  to  show  how  the  student  of  astrophysics  attacks  this 
problem  of  stellar  evolution,  through  the  development  of 
special  instruments  and  methods  of  research,  and  the  accu- 
mulation and  discussion  of  observations. 

It  must  not  be  forgotten  that  such  a  study  comprises  only 
the  earliest  and  simplest  elements  in  the  general  problem  of 
evolution.  The  province  of  the  student  of  astrophysics  may 
be  said  to  end  with  an  understanding  of  the  production  of  a 
planet  like  the  Earth.  It  remains  for  the  geologist  to  explain 
the  ^changes  which  the  surface  of  the  Earth  has  undergone 
since  the  constructive  process  left  it  a  rocky  crust.  The  con- 
ditions which  brought  about  the  formation  of  the  oceans,  the 
effects  of  the  long-continued  action  of  winds  and  waves,  and 
the  vast  changes  in  surface  structure  that  have  resulted  from 
internal  disturbances  and  the  operation  of  volcanic  phenom- 
ena, afford  limitless  opportunity  to  the  student  of  evolution 
in  this  other  aspect.  Closely  related  to  these  changes,  and 
presenting  difficulties  far  greater  than  those  experienced  by 
the  astrophysicist,  comes  the  problem  of  accounting  for  the 
origin  and  development  of  plant  and  animal  life.  The  pres- 
ervation of  the  earlier  forms  of  life,  principally  through  the 
agency  of  sedimentary  deposits,  affords  the  paleontologist  the 
means  of  connecting  the  links  in  the  evolutionary  chain. 
Thus  we  are  brought  to  our  own  era,  where  countless  living 
objects  continue  to  supply  material  for  new  inquiries.  Both 
in  the  examination  of  existing  species  and  their  relationships, 
and  in  those  experimental  researches  on  variation  which  offer 


THE  PROBLEM  OF  STELLAR  EVOLUTION     5 

such  promising  opportunities  to  the  investigator,  the  evolu- 
tionist may  secure  data  for  further  advances.  Outside  the 
immediate  domain  of  the  natural  sciences,  in  regions  of 
activity  where  still  greater  complexity  prevails,  the  student 
may  seek  to  trace  out  evidences  of  unity  and  development  in 
the  mental  and  moral  relationships  of  the  peoples  of  many 
countries  and  of  many  generations. 

It  is  a  noteworthy  fact,  of  prime  significance  to  all  investi- 
gators who  find  special  interest  in  attempting  to  enter  new 
and  unoccupied  fields,  that  some  of  the  most  important  devel- 
opments of  recent  years  have  taken  place  in  those  regions 
which  lie  between  the  boundaries  of  the  old  established 
sciences.  Thus  the  union  of  physics  and  chemistry  has 
opened  up  the  extensive  field  of  physical  chemistry,  where 
advances  of  the  greatest  value  are  being  made.  In  the  same 
way  the  application  of  physical  methods  and  the  principles 
of  physical  chemistry  to  the  experimental  study  of  physiology 
has  resulted  so  successfully  as  to  give  hope  for  even  more 
remarkable  developments  in  the  near' future.  In  astronomy, 
the  introduction  of  physical  methods  has  revolutionized  the 
observatory,  transforming  it  from  a  simple  observing  station 
into  a  laboratory,  where  the  most  diverse  means  are  employed 
in  the  solution  of  cosmical  problems.  The  fact  that  physics 
is  common  to  these  and  other  intermediate  branches  of 
science  affords  striking  proof  of  its  fundamental  importance. 
An  investigator  who  has  been  confined  to  the  traditional 
methods  of  a  department  of  science  where  physics  has  as  yet 
played  little  part,  may  therefore  find  in  physical  methods  a 
powerful  means  of  advancing  his  subject. 

The  suggestive  value,  to  investigators  in  other  depart- 
ments, of  any  species  of  scientific  research  which  involves  new 
methods  and  principles  is  perhaps  greater  at  the  present 
time  than  ever  before.  Even  those  methods  of  research 
which  can  find  no  direct  application  in  other  subjects  are 


6  STELLAR  EVOLUTION 

frequently  capable  of  suggesting  modifications  or  adaptations 
involving  related  principles.  The  development  and  use  of 
new  methods  is  quite  as  likely  to  advance  a  subject  as  the 
prosecution  of  extensive  investigations  by  existing  means. 
For  this  reason  the  investigator  is  ever  on  the  alert  to  seize 
and  utilize  suggestions  derived  from  any  source. 

The  interest  of  the  student  of  astrophysics  is  no  longer 
confined  simply  to  celestial  phenomena.  For  astrophysics 
has  become,  in  its  most  modern  aspect,  almost  an  experi- 
mental science,  in  which  some  of  the  fundamental  problems 
of  physics  and  chemistry  may  find  their  solution.  The  stars 
may  be  regarded  as  enormous  crucibles,  in  some  of  which 
terrestrial  elements  are  subjected  to  temperatures  and  pres- 
sures far  transcending  those  obtainable  by  artificial  means. 
In  the  Sun,  which  appears  to  us  not  merely  as  a  point  of 
light  like  the  stars,  but  as  a  vast  globe  whose  every  detail 
can  be  studied  in  its  relationship  to  the  general  problem  of 
the  solar  constitution,  the  immense  scale  of  the  phenomena 
always  open  to  observation,  the  rapidity  of  the  changes,  and 
the  enormous  masses  of  material  involved,  provide  the  means 
for  researches  which  could  never  be  undertaken  in  terrestrial 
laboratories.  Hence  it  is  that  astrophysics  may  equally  well 
be  regarded  as  a  branch  of  physics  or  as  a  branch  of  astron- 
omy. A  telescope  may  be  defined  as  an  instrument  for 
revealing  celestial  phenomena,  or  it  may  be  likened  to  the 
lens  which  the  physicist  uses  in  his  laboratory  to  concentrate 
the  light  of  an  electric  spark  on  the  slit  of  his  spectroscope. 
To  the  student  of  astrophysics  whose  interests  are  not  con- 
fined to  a  single  branch  of  science,  the  subject  is  likely  to 
make  a  double  appeal,  no  less  strong  on  the  physical  and 
chemical  than  on  the  astronomical  side. 

In  entering  upon  our  consideration  of  the  study  of  stellar 
development,  we  may  think  of  the  subject  in  either  one  of 
two  ways.  Some  will  prefer  to  regard  it  as  the  general  prob- 


THE  PROBLEM  OF  STELLAR  EVOLUTION     7 

lem  of  stellar  evolution,  in  its  broad  application  to  the  uni- 
verse at  large.  But  others  will  find  it  easier  to  conceive  of 
the  question  as  an  investigation  of  the  Sun,  tracing  it, 
through  analogies  afforded  by  stars  in  earlier  stages  of 
growth,  from  its  origin  in  a  nebula  to  those  final  chapters 
which,  though  not  yet  written  for  the  Sun  itself,  may  be  read 
in  the  life-histories  of  the  red  stars.  Viewed  from  which- 
ever standpoint,  the  task  of  the  investigator  remains  the 
same,  since  in  either  case  it  is  concerned  with  stellar  origin, 
development,  and  decay. 

It  must,  of  course,  be  remembered  that  the  processes  of 
stellar  development  ordinarily  advance  so  slowly  that  a  life- 
time would  be  far  too  short  to  permit  any  permanent  change 
to  be  observed  in  a  star.  Temporary  stars  flash  into  view  and 
fade  rapidly  away;  but  these  represent  an  abnormal  condi- 
tion, typical  of  some  catastrophe  rather  than  of  a  natural 
course  of  change.  The  spiral  nebulae,  though  their  appear- 
ance leaves  little  doubt  of  extremely  rapid  motion  and  con- 
stant change  of  form,  are  so  far  removed,  and  constructed  on 
so  vast  a  scale,  that  no  actual  differences  in  structure  have 
been  detected  in  photographs  of  the  same  object,  taken  at 
intervals  of  many  years.  In  the  processes  of  creation  a  thou- 
sand years  is  but  a  day,  and  we  must  be  content  to  base  our 
stellar  histories  upon  analogy. 

Fortunately,  the  data  needed  for  the  construction  of  these 
histories  are  easily  found.  Our  problem  is  like  that  of  one 
who  enters  a  forest  of  oaks,  and  desires  to  learn  through 
what  stages  the  trees  have  passed  in  reaching  their  present 
condition.  He  cannot  wait  long  enough  to  see  any  single 
tree  go  through  its  long  cycle  of  change.  But  on  the  ground 
he  may  find  acorns,  some  unbroken  and  some  sprouting. 
Others  have  given  rise  to  rapidly  growing  shoots,  and  sap- 
lings are  at  hand  to  show  the  next  stage  of  growth.  From 
saplings  to  trees  is  an  easy  step.  Then  may  be  found,  in  the 


8  STELLAR  EVOLUTION 

form  of  dead  limbs  and  branches,  the  first  evidences  of  decay, 
reaching  its  full  in  fallen  trunks,  where  the  hard  wood  is 
wasting  to  powder. 

Scattered  over  the  heavens  are  millions  of  stars,  each 
representing  a  certain  degree  of  development.  The  cloud 
forms  of  the  nebulae  tell  us  of  stellar  origins;  the  white, 
yellow,  and  red  stars  illustrate  the  rise  and  decline  of  stellar 
life;  and  the  Earth  itself  affords  a  picture  of  what  may 
remain  after  light  and  heat  have  been  extinguished. 


CHAPTER  II 

THE  STUDENT  OP  THE  NEW  ASTRONOMY 

THE  traditional  conception  of  the  astronomer,  while  still 
applicable  (with  sundry  limitations)  in  certain  modern 
instances,  does  not  accurately  apply  to  the  student  of  stellar 
evolution.  According  to  the  old  view,  the  astronomer,  soon 
after  the  setting  of  the  Sun,  retires  to  a  lofty  tower,  from 
whose  summit  he  gazes  at  the  heavens  throughout  the 
long  watches  of  the  night.  His  eye,  fixed  to  the  end  of  a 
telescope  tube,  perceives  wonders  untold,  while  his  mind 
sweeps  with  his  vision  through  the  very  confines  of  the  uni- 
verse. The  lineal  descendant  of  the  seers  and  soothsayers 
of  the  Chaldeans,  he  dwells  apart,  finding  little  of  interest  in 
the  ordinary  concerns  of  the  world,  so  occupied  are  his 
thoughts  with  celestial  mysteries. 

Now  there  can  be  no  doubt  that  the  study  of  stellar  evo- 
lution brings  a  degree  of  pleasure  and  enthusiasm  which  it 
would  be  difficult  to  surpass.  The  joys  of  the  pioneer,  the 
excitement  that  comes  to  him  who  looks  for  the  first  time 
upon  an  unknown  land,  the  intense  satisfaction  of  discovery, 
all  belong  to  the  successful  investigator.  Moreover,  mere 
gazing  through  a  telescope,  as  distinguished  from  the  pains- 
taking work  of  modern  astronomers  with  micrometer  or 
photographic  plate,  is  still  competent  to  reveal  new  or  chan- 
ging phenomena,  and  important  discoveries  are  yet  to  come 
to  the  alert  and  careful  observer.  It  is  pleasant  to  picture 
the  surprise  and  delight  of  Galileo  when  he  first  perceived 
spots  on  the  supposedly  immaculate  surface  of  the  Sun.  His 
little  instrument,  much  less  perfect  than  a  modern  spy -glass, 
could  reveal  none  of  that  intricate  structure  and  exquisite 

9 


10  STELLAK  EVOLUTION 

detail  that  are  at  once  the  joy  and  the  despair  of  present-day 
sun-spot  observers.  But  he  had  discovered  a  new  and 
important  fact,  the  basic  principle  of  the  science  of  astro- 
physics: he  had  shown  that  with  suitable  optical  aid  the 
physical  structure  of  the  heavenly  bodies  might  be  investi- 
gated. Prior  to  this  time  astronomy  had  concerned  itself 
only  with  the  positions  and  motions  of  the  stars;  now  it 
became  evident  that  each  of  these  luminaries  might  present 
peculiar  and  distinguishing  phenomena  worthy  of  the  most 
searching  investigation.  Discovery  followed  discovery  in 
rapid  sequence.  The  mottled  face  of  the  Moon,  formerly 
without  meaning,  was  suddenly  revealed  in  unsuspected 
landscapes  of  valley,  plain,  and  mountain,  resembling,  in 
curious  degree,  the  variegated  surface  of  the  Earth.  Jupiter, 
who  had  seemed  to  travel  alone  through  the  heavens,  was 
found  to  possess  four  companions,  whose  revolutions  about 
him  forcibly  suggested  the  revolutions  of  the  planets  about 
the  Sun.  The  mysterious  ansae,  inclosing  between  them 
the  globe  of  Saturn,  were  soon  made  out  to  be  the  more 
conspicuous  elements  of  a  vast  incircling  ring,  unlike  any- 
thing of  earlier  experience.  With  the  growth  of  the  tele- 
scope more  marvels  were  brought  to  light,  until  it  seemed,  in 
sound  reason,  as  though  the  universe  would  never  cease  to 
yield  new  knowledge  to  the  explorer  of  its  boundless  wastes. 
Thus  was  established  that  conception  of  the  astronomer 
that  still  persists,  long  after  a  new  astronomy  has  come  into 
being.  Gazing  through  a  telescope,  as  has  been  said,  is  still 
competent  to  bring  discoveries ;  for  change  is  the  very  essence 
of  celestial  phenomena,  and  persistent  watching  must  detect 
important  facts,  on  which  broad  generalizations  may  be 
founded.  But  the  eye  and  the  telescope  have  been  supple- 
mented by  various  instrumental  aids  which,  in  their  multi- 
plication, have  transformed  the  occupations  of  the  astronomer. 
The  micrometer,  in  its  application  to  the  accurate  measure- 


THE  STUDENT  OF  THE  NEW  ASTRONOMY         11 

ment  of  place  and  form,  permits  changes  to  be  detected  which 
are  beyond  the  perception  of  the  eye.  The  photometer,  in 
its  precise  determinations  of  brightness,  has  shown  that  stars 
whose  light  never  varies  are  rather  the  exception  than  the  rule. 
The  photographic  plate,  used  in  conjunction  with  the  tele- 
scope, has  proved  itself  to  be  more  sensitive  than  the  human 
retina,  in  that  it  is  capable  of  adding  up  into  a  visible  record 
the  invisible  radiations  received  during  an  exposure  of  many 
hours.  Finally,  to  mention  but  one  more  of  the  telescope's 
new  adjuncts,  the  spectroscope  has  introduced  a  new  and 
revolutionary  principle  into  astronomy,  permitting  the  chemi- 
cal and  physical  analysis  of  the  most  distant  stars. 

Hence  it  is  that  the  present-day  student  of  astrophysics 
does  not  correspond  to  the  traditional  idea  of  the  astronomer. 
His  work  at  the  telescope  is  largely  confined  to  such  tasks  as 
keeping  a  star  at  the  precise  intersection  of  two  cross-hairs, 
or  on  the  narrow  slit  of  a  spectrograph,  in  order  that  stars 
and  nebulae,  or  their  spectra,  may  be  sharply  recorded  upon 
the  photographic  plate.  His  most  interesting  work  is  done, 
and  most  of  his  discoveries  are  made,  when  the  plates  have 
been  developed,  and  are  subjected  to  long  study  and  measure- 
ment under  the  microscope.  His  problems  of  devising  new 
methods  of  calculation  or  reduction  are  as  fascinating  as  the 
invention  of  new  instruments  of  observation.  Much  of  his 
time  may  be  spent  in  the  laboratory,  imitating,  with  the  means 
placed  at  his  disposal  by  the  physicist  and  chemist,  the  vari- 
ous conditions  of  temperature  and  pressure  encountered  in 
the  stars,  and  watching  the  behavior  of  metals  and  gases  in 
these  uncommon  environments.  If,  in  the  conviction  that  new 
and  promising  means  of  research  are  always  awaiting  applica- 
tion, he  would  advance  into  still  unoccupied  fields,  he  must 
devote  himself  to  the  design  and  construction  of  new  instru- 
ments, to  supplement  the  old.  Kept  thus  in  touch  with  the 
newest  phases  of  physical  and  chemical  investigation,  the 


12  STELLAE  EVOLUTION 

countless  applications  of  electricity,  the  methods  of  modern 
engineering,  and  the  practical  details  of  workshop  practice, 
his  interest  in  these  things  of  the  world  is  likely  to  be  quite 
as  broad  as  that  of  the  average  man.  His  sympathy  with 
research  in  every  branch  of  science  must  increase  and 
strengthen  as  his  conception  of  the  great  problem  of  evolu- 
tion is  developed  by  his  own  investigations  of  its  earliest 
phases.  And  the  pleasure  and  enthusiasm  derived  from  his 
studies  must  become,  not  like  the  vague  passion  of  the  mys- 
tic, whose  inability  to  see  clearly  leads  him  to  pursue  strange 
gods,  but  such  as  every  successful  searcher  after  truth  must 
experience,  whether  he  deal  with  the  vast  dimensions  and 
distances  of  the  heavenly  bodies,  or  with  the  minute  but  no 
less  marvelous  phenomena  of  microscopic  life  and  form. 

Now,  while  it  cannot  be  too  strongly  emphasized  that  the 
student  of  stellar  evolution  can  have  no  sympathy  with  the 
mystic,  whose  habit  of  thought  must  be  the  very  antithesis 
of  his  own,  yet  it  is  true  that  the  imagination,  when  properly 
exercised  and  controlled,  is  to  be  regarded  as  his  best  aid  to 
progress.  The  question  of  control  is  so  important  that  it 
may  well  be  mentioned  first.  For  nothing  has  done  more 
injury  to  science  than  the  play  of  imaginations  subject  to  no 
control,  on  the  part  of  men  who  enjoy  in  the  public  press  the 
rank  of  scientific  authorities.  Thus  great  sun-spots  become 
the  innocent  cause  of  earthquakes  or  tornadoes,  not  to  speak 
of  their  effect  upon  the  price  of  wheat.  Comets,  once  the 
unerring  portents  of  war  and  pestilence,  still  carry  the  brands 
of  conflagration,  and  threaten  at  each  apparition  to  destroy 
the  Earth.  Mystic  properties  are  ascribed  to  the  center  of 
the  universe,  and  a  well-known  planet,  because  it  is  incor- 
rectly assumed  to  be  stationed  there,  is  dogmatically  asserted 
to  be  the  only  possible  abode  of  human  life.  There  is  a  fine 
field  here  for  humor  and  amusing  speculation,  as  the  author 
of  the  "Moon  Hoax,"  and  other  more  recent  writers,  have 


THE  STUDENT  OF  THE  NEW  ASTRONOMY         13 

shown  us.  But  humor  is  not  always  intended:  the  pronun- 
ciamentos  go  forth  in  the  name  of  science,  and  are  so  accepted 
by  a  host  of  intelligent  persons,  who  naturally  believe  that  the 
supposed  authorities  have  reached  their  conclusions  by  scien- 
tific methods.  Thus  there  arises  a  false  conception  of  science, 
and  a  popular  demand  for  wonders,  which  is  not  easily 
satisfied  by  acquaintance  with  the  less  sensational  facts. 

But  though  dangerous  when  unrestrained,  the  imagina- 
tion, when  rightly  exercised,  is  the  best  guide  of  the 
astronomer.  His  dreams  run  far  ahead  of  his  accomplish- 
ments, and  his  work  of  today  is  part  of  the  development  of 
a  plan  projected  years  ago.  He  perceives  that  only  a  few 
generations  hence  many  of  the  instruments  and  methods  of 
his  time  are  to  be  replaced  by  better  ones,  and  he  strains  his 
vision  to  obtain  some  glimpse,  imperfect  though  it  be,  into 
the  obscurities  of  the  future.  As  he  sits  in  his  laboratory, 
surrounded  by  lenses  and  prisms,  gratings  and  mirrors,  and 
the  other  elementary  apparatus  of  a  science  that  subsists  on 
light,  he  cannot  fail  to  entertain  the  alluring  thought  that 
the  intelligent  recognition  of  some  well-known  principle  of 
optics  might  suffice  to  construct,  from  these  very  elements, 
new  instruments  of  enormous  power.  He  learns  of  some 
advance  in  engineering  or  in  the  art  of  the  glass-maker,  and 
dreams  of  new  possibilities  in  its  application  to  the  construc- 
tion of  his  telescopes  or  the  equipment  of  his  laboratory.  He 
reads  of  discoveries  in  physics  or  chemistry,  and  at  once  his 
mind  is  busy  in  its  endeavor  to  apply  the  new  knowledge  to 
the  solution  of  long-standing  cosmical  problems. 

But  here,  again,  we  see  the  need  of  control ;  for  with  such 
a  multiplicity  of  interests,  and  such  constant  stimulus  to  the 
imagination,  the  danger  of  mere  dilettantism  is  obvious. 
With  scores  of  problems  suggesting  themselves  for  solution, 
and  with  attractions  on  every  hand,  each  rivaling  the  other 
in  its  apparent  possibilities  of  development,  the  chief  difficulty 


14  STELLAR  EVOLUTION 

is  to  choose  wisely.  It  is  not  a  question  of  searching  for 
something  to  do,  but  of  picking  out  those  things  which  are 
most  worthy  of  pursuit.  Here  the  importance  of  having  a 
definite  and  logical  plan  of  research  becomes  apparent.  Such 
a  plan  may  involve  a  single  investigation,  continued  along 
systematic  lines  over  a  long  period  of  years,  or  it  may  com- 
prise several  investigations,  carried  on  simultaneously.  In 
a  large  observatory  each  piece  of  work  acquires  increased 
importance  if  it  is  selected,  not  at  random,  or  solely  because 
of  its  intrinsic  value,  but  rather  because  of  the  part  it  plays 
in  a  single  logical  scheme  of  research.  Its  intrinsic  impor- 
tance need  not  be  in  the  least  diminished  by  its  relationship 
to  other  work,  while  the  illumination  which  its  results  cast  on 
the  other  investigations  of  the  scheme  can  hardly  fail  to 
improve  them,  and  may  even- reveal  the  chief  source  of  their 
meaning.  Moreover,  the  same  research,  if  carried  on  else- 
where, might  prove  of  small  value,  in  the  absence  of  such 
suggestions  and  modifications  as  are  sure  to  come  from  the 
related  investigations.  We  shall  have  occasion  to  revert  to 
this  question  in  discussing  a  plan  of  attack  on  the  general 
problem  of  stellar  evolution. 


CHAPTER   III 
THE  SUN  AS  A  TYPICAL  STAR 

BEFOKE  proceeding  to  the  more  detailed  portions  of  our 
discussion,  let  us  examine  the  present  condition  of  the  bodies 
with  which  we  are  to  deal,  and  briefly  trace  out  those  ele- 
ments of  relationship  which  it  will  be  our  purpose  later  to 
describe  more  fully.  Let  us  begin  with  the  consideration  of 
a  single  object,  which  we  may  afterward  compare  with  other 
objects  less  easily  observed  because  of  their  greater  distance 
from  the  Earth. 

The  photographic  reproduction  in  Plate  II  represents  the 
Sun,  as  seen  with  an  ordinary  telescope.  So  far  as  could  be 
judged  from  this  picture,  the  Sun  might  be  described  as  a 
luminous  sphere,  brighter  in  its  central  part  than  near  its 
circumference,  and  marked  with  dark  spots,  irregularly  dis- 
tributed over  the  surface.  On  closer  examination  it  will  also 
be  seen  that  there  are  certain  bright  regions,  which  are  most 
easily  noticed  near  the  edge  of  the  Sun.  The  dark  spots  are 
the  well-known  sun-spots,  first  discovered  by  Galileo,  while 
the  bright  regions  are  the  faculae,  which  have  also  been 
known  since  the  invention  of  the  telescope^.  At  times  of  total 
eclipse,  when  the  bright  body  of  the  Sun  is  covered  by  the 
dark  body  of  the  Moon,  shielding  our  atmosphere  from  the 
usual  brilliant  illumination,  red  flames,  sometimes  reaching 
heights  of  several  hundred  thousand  miles,  may  be  seen  rising 
from  a  continuous  sea  of  flame,  which  completely  incircles 
the  Sun.  These  are  the  prominences,  and  the  continuous 
mass  of  flame  from  which  they  rise  is  the  chromosphere 
(Plate  III)  .*  Extending  far  beyond  these  flames  into  space, 

1  See  the  remarks  on  anomalous  dispersion,  p.  148. 

15 


16  STELLAK  EVOLUTION 

sometimes  to  a  distance  of  millions  of  miles,  is  the  corona, 
which  shines  with  a  silvery  luster  somewhat  inferior  in  bright- 
ness to  that  of  the  full  Moon  (Fig.  2,  Plate  IV). 

An  analysis  of  the  light  of  the  Sun,  made  with  the  spectro- 
scope, has  shown  the  presence  of  the  vapors  of  iron,  sodium, 
magnesium,  calcium,  hydrogen,  and  many  other  substances 
known  to  us  on  the  Earth.  In  fact,  jt  has  been  remarked 
that  if  the  Earth  were  heated  to  the  temperature  of  the  Sun, 
the  light  emitted  by  its  vapors  would  resemble  closely,  when 
analyzed  with  the  spectroscope,  the  light  emitted  by  the  Sun. 
Thus  the  chemical  composition  of  the  Earth  and  the  Sun  is 
much  the  same,  although  we  have  evidence  of  the  existence 
in  the  Sun  of  a  large  number  of  substances  not  yet  found  on 
the  Earth.  This  same  means  of  analysis  has  led  to  the  dis- 
covery that  the  chromosphere,  and  the  prominences  which 
rise  out  of  it,  are  composed  of  the  vapor  of  calcium  and  of  the 
light  gases  helium  and  hydrogen.  The  sun-spots,  too,  have 
also  been  found  to  have  a  characteristic  chemical  compo- 
sition; while  the  corona  emits  rays  which  probably  indicate 
the  presence  in  it  of  very  light  and  tenuous  gases. 

Observations  of  the  Sun.  continued  without  interruption 
for  more  than  half  a  century,  have  shown  that  the  spots  are 
not  constant  in  number,  but  vary  in  a  characteristic  way  in 
a  period  of  about  eleven  years.  At  times  of  sun-spot  maxi- 
mum the  surface^of  the  Sun  is  marked  by  large  numbers 
of  spots,  which  are  found  on  attentive  observation  to  be  the 
scene  of  great  activity,  and  frequently  the  source  of  the  most 
violent  eruptions.  At  this  period  the  prominences  are  large 
and  abundant,  and  testify  to  the  general  condition  of  disturb- 
ance by  exhibiting,  from  time  to  time,  eruptive  phenomena 
on  a  very  large  scale,  in  which  great  masses  of  gas  have 
been  known  to  shoot  upward  with  velocities  of  hundreds  of 
miles  a  second.  With  the  passage  of  time  these  evidences  of 
disturbance  and  activity  become  less  and  less  marked,  until 


THE  SUN  AS  A  TYPICAL  STAR  17 

finally,  during  the  minimum  period,  the  surface  of  the  Sun 
for  months  together  may  be  wholly  devoid  of  sun-spots. 
The  prominences  also  become  less  numerous,  and  eruptive 
phenomena,  so  common  during  the  maximum  period,  are 
rarely  to  be  observed  at  the  minimum.  Even  the  corona 
undergoes  changes  in  form  which  are  perfectly  charac- 
teristic, and  show  a  definite  connection  with  the  sun-spot 
period. 

So  much  for  the  Sun  and  its  more  conspicuous  phenomena. 
We  are  now  led  to  inquire  whether  it  has  any  counterparts 
among  the  other  heavenly  bodies.  Let  us. suppose  the  Sun 
removed  to  the  distance  of  the  nearest  fixed  stars.  Its  light 
would  then  be  reduced  in  so  great  a  degree  as  to  be  sur- 
passed by  that  of  many  of  the  brighter  stars,  though  it 
would  still  remain  one  of  the  more  conspicuous  objects  in 
the  heavens.  The  planets  of  the  solar  system  would  be 
wholly  beyond  the  range  of  observation,  even  with  the  most 
powerful  telescopes.  The  light  of  the  Sun  would  appear 
yellowish,  and  it  would  be  impossible  to  distinguish  it  from 
certain  stars  which  also  shine  with  a  yellowish  light.  Spec- 
troscopic  analysis  of  the  light  of  these  stars  reveals  the 
presence  in  their  atmospheres  of  elements  familiar  to  us  on 
the  Earth;  indeed,  the  chemical  composition  of  some  of 
them  can  be  shown  to  be  practically  identical  with  that  of  the 
Sun.  On  account  of  its  immense  distance,  the  Sun's  disk 
would  be  reduced  to  a  minute  point  of  light,  as  in  the  case  of 
the  other  stars,  and  the  sun-spots,  prominences,  corona,  and 
other  phenomena  would  be  wholly  invisible.  For  the  same 
reason,  such  phenomena,  though  undoubtedly  present  in 
other  stars,  are  hidden  from  observation.  We  may  there- 
fore conclude  that  the  Sun  is  a  star,  practically  identical  in 
chemical  composition  and  in  physical  constitution  with  many 
other  stars  in  the  heavens,  and  ranking  in  size  below  many 
of  these  objects. 


18  STELLAR  EVOLUTION 

A  very  casual  acquaintance  with  the  stars,  based  upon 
naked-eye  observations,  is  sufficient  to  make  one  familiar 
with  the  fact  that  they  differ  from  each  other  as  much  in 
color  as  they  do  in  brightness.  Such  objects  as  Sirius  shine 
with  a  bluish-white  light,  whereas  Arcturus  is  yellowish  like 
the  Sun.  Antares,  in  the  Scorpion,  is  a  fine  example  of  a 
red  star,  and  with  the  telescope  smaller  stars  may  be  seen 
of  a  deeper  red  color.  Spectroscopic  study  of  these  various 
classes  of  stars  shows  in  the  clearest  way  definitive  peculiari- 
ties, which  may  form  the  basis  of  a  system  of  classification. 
Indeed,  we  apparently  find  ourselves  in  the  presence  of  stars 
in  every  stage  of  growth,  from  the  earliest,  as  represented 
by  the  bluish-white  objects,  to  the  latest,  typified  by  the  red 
stars1  (Fig.  1,  Plate  IV).  Intermediate  in  point  of  develop- 
ment are  yellowish  stars  like  the  Sun. 

In  various  parts  of  the  heavens  clusters  may  be  observed, 
in  some  of  which  the  stars  are  widely  scattered,  as  in  the 
Pleiades,  while  in  others  they  are  densely  packed  together, 
so  closely  that  several  thousand  stars  may  sometimes  be  seen 
within  an  area  so  small  that  to  the  naked  eye  they  appear  like 
a  single  hazy  star.  Since  we  find  clusters  of  every  degree  of 
density,  and  since  the  stars  in  the  heart  of  some  of  these 
clusters  are  too  close  together  to  be  separated  by  the  tele- 
scope, the  question  long  ago  arose  whether  the  nebulae, 
which  seem  to  resemble  luminous  clouds  in  the  heavens,  are 
to  be  regarded  as  star  clusters  so  dense  as  to  be  beyond 
telescopic  resolution.  It  was  not  until  the  spectroscope  had 
been  applied  by  Huggins  (see  p.  54)  that  this  question  was 
finally  settled.  It  then  appeared  that  some  of  the  nebulae,  at 
least,  are  vast  masses  of  luminous  gas,  and  that  they  are 
therefore  not  composed  of  separate  stars.  It  might  then  be 
inquired  what  part  in  the  scheme  of  evolution  such  nebulae 
play.  It  will  be  shown  in  the  course  of  this  book  that  there 

1  See  the  cautionary  remarks  on  p.  198. 


THE  SUN  AS  A  TYPICAL  STAR  19 

exists  between  stars  and  nebulae  a  relationship  so  intimate 
as  to  leave  little  doubt  that  stars  are  condensed  out  of  nebulae 
through  the  long-continued  action  of  gravitation.  It  thus 
seems  probable  that  the  nebulae  represent  the  stuff  from 
which  stars  are  made,  in  its  primitive  and  uncondensed 
state. 


CHAPTER  IV 

-v 

LARGE  AND  SMALL  TELESCOPES 

IT  must  soon  appear,  to  one  who  seeks  in  the  heavens 
with  unaided  vision  for  evidences  of  stellar  evolution,  that 
but  little  progress  can  be  made  without  powerful  instrumental 
means.  When  the  nature  of  the  problem  is  considered,  and 
it  is  remembered  that  all  observations  of  the  stars  must  be 
made  from  the  surface  of  a  minute  body  moving  through  the 
midst  of  the  universe,  the  only  cause  for  surprise  will  be  that 
instruments  of  sufficient  power  for  our  purpose  can  be  con- 
structed. The  distances  of  the  stars  are  so  enormous  that  it 
might  seem  hopeless  ever  to  solve  the  problem  of  their  phys- 
ical constitution,  or  to  analyze  them  as  the  chemist  resolves 
into  its  elements  a  substance  in  his  laboratory. 

Let  us  consider  what  must  be  accomplished  before  we  may 
even  begin  to  study  the  subject  of  stellar  evolution.  In  the 
course  of  our  work  we  must  deal  with  stars  which  are  not  only 
invisible  to  the  naked  eye,  but  are  beyond  the  reach  of  any 
except  the  most  powerful  telescopes.  We  must  find  the  means 
of  collecting  the  light  from  such  bodies,  not  only  those  rays 
which,  if  intense  enough,  could  be  seen  by  the  eye,  but 
also  those  which,  because  of  the  structure  of  the  eye,  are 
wholly  invisible.  After  collecting  together  such  rays,  we 
must  subject  them  to  analysis  by  instruments  which  will  per- 
mit us  to  draw  conclusions,  both  as  to  the  nature  of  the 
chemical  elements  present  in  the  star's  atmosphere  and  as  to 
the  physical  condition  of  these  elements,  illustrated  by  the 
pressure  and  the  temperature  to  which  they  are  subjected. 
Although  we  may  never  hope  to  see  a  star's  actual  disk, 
even  in  the  most  powerful  telescopes  of  the  future,  as  other 

20 


LARGE  AND  SMALL  TELESCOPES       21 

than  a  minute  point  of  light,  we  must  find  means  of  differ- 
entiating one  part  of  the  star  from  another  and  of  determin- 
ing, for  example,  whether  the  vapor  of  carbon  lies  above 
or  below  that  of  iron  or  sodium  in  its  atmosphere.  If 
luminous  clouds,  like  those  on  the  Sun,  are  strikingly  char- 
acteristic of  the  star  under  observation,  we  must  be  able  to 
detect  their  presence,  though  we  may  never  see  their  form. 
If,  as  in  the  case  of  temporary  stars,  vast  temperatures  or 
pressures  may  produce  great  differences  in  physical  condition 
between  the  inner  and  outer  parts  of  a  stellar  atmosphere, 
we  must  learn  a  way  of  discovering  such  differences  and  of 
ascribing  them  to  their  true  cause.  Incidentally,  and  as  a 
necessary  precedent  to  these  studies,  we  must  be  able  to 
determine  whether  the  star  is  moving  toward  or  away  from 
the  Earth,  and  to  measure  its  velocity  in  either  direction  with 
great  precision. 

Moreover,  our  means  of  analysis  must  be  so  refined  that 
they  shall  enable  us  to  investigate,  not  merely  the  general 
physical  and  chemical  properties  of  single  stars,  but  also 
those  minute  peculiarities  of  composition  or  of  motion  which 
may  relate  them  to  other  stars,  and  define  their  precise  place 
in  some  general  scheme  of  stellar  evolution.  We  must  have 
some  means  at  hand  which  will  bring  to  light  the  forms  of 
nebulae,  even  though  they  be  invisible  to  a  trained  eye  aided 
by  the  most  powerful  telescope  ever  constructed.  Being 
given  these  forms,  we  must  seek  for  evidences  of  relationship 
between  the  cloudlike  nebulae  and  the  stellar  points  which 
they  surround.  And  the  means  of  analysis  which  tells  us  of 
the  constitution  of  the  stars  must  also  tell  us  of  the  nature  of 
the  nebulae,  thus  serving  to  establish  relationships  with  stars 
which  no  mere  indications  of  position  or  of  structure  could 
provide. 

A  refracting  telescope  consists  of  a  lens  (object-glass) 
usually  mounted  at  the  end  of  a  long  tube,  which  is  pointed 


22  STELLAK  EVOLUTION 

at  the  object  to  be  observed.  In  the  present  case  we  will 
suppose  this  to  be  the  Moon.  The  lens  forms  an  image  of 
the  Moon  at  the  lower  end  of  the  tube,  just  as  the  lens  of  a 
camera  forms  an  image  on  the  ground-glass.  Indeed,  a  tele- 
scope may  be  regarded  as  nothing  more  or  less  than  a  long 
camera,  in  which  a  tube  is  substituted  for  the  ordinary  bellows. 
By  putting  a  plate  at  the  point  where  the  image  is  formed,  and 
giving  a  suitable  exposure,  the  Moon  may  be  photographed, 
just  as  a  landscape  is  photographed  with  the  camera.  For 
eye  observations,  however,  the  image  formed  by  the  telescope 
is  looked  at  through  a  small  lens  called  an  eye-piece.  The 
image  is  magnified  in  the  same  way,  and  to  the  same  extent, 
as  any  object  would  be  if  looked  at  with  the  eye-piece,  used 
as  an  ordinary  hand  magnifier.  The  total  magnifying  power 
of  the  telescope,  however,  of  course  depends  not  only  upon 
the  magnifying  power  of  the  eye-piece,  but  also  upon  the  size 
of  the  image  formed  by  the  object-glass.  The  size  of  this 
image  is  determined  solely  by  the  focal  length,  or  distance 
from  the  object-glass  to  the  image.  Suppose,  for  example, 
we  have  two  telescopes,  with  object-glasses  of  the  same  diam- 
eter, but  of  different  focal  lengths.  The  one  of  longer  focal 
length  will  give  the  larger  image.  If  the  focal  length  is  twice 
that  of  the  other  telescope,  the  image  will  be  twice  as  large. 
With  the  same  eye-piece,  therefore,  the  magnifying  power  of 
the  longer  telescope  will  be  twice  that  of  the  shorter  one. 

We  thus  see  that  the  size  of  the  image  given  by  a  tele 
scope  does  not  depend  upon  the  diameter  of  its  object-glas^. 
The  brightness  of  the  image,  however,  evidently  does  depend 
upon  the  amount  of  light  concentrated  in  it,  and  this  increases 
with  the  diameter  of  the  object-glass.  If  we  double  the 
diameter  of  the  object-glass,  we  get  four  times  as  much  light 
in  the  image  of  a  star;  for  the  amount  of  light  collected 
depends  upon  the  area  of  the  object-glass,  and  this  increases 
as  the  square  of  its  diameter. 


LARGE  AND  SMALL  TELESCOPES  23 

These  details  are  worth  remembering,  for  they  determine, 
in  great  measure,  the  relative  advantages  of  large  and  small 
telescopes.  There  is  another  consideration,  however,  of  the 
first  importance,  which  must  not  be  overlooked.  A  small 
telescope  is  limited,  by  the  very  nature  of  light,  in  its  power 
of  separating  two  closely  adjacent  stars.  If  these  stars  are 
less  than  a  certain  distance  apart,  no  increase  in  the  magnify- 
ing power  of  the  telescope,  either  through  increase  in  its  focal 
length  or  through  the  use  of  a  more  powerful  eye-piece,  can 
possibly  show  them  as  separate  objects.  The  reason  lies  in 
the  fact  that  the  image  of  a  star  in  a  telescope  is  a  minute 
disk,  the  diameter  of  the  disk  depending  on  the  size  of  the 
object-glass.  The  disk  grows  smaller  as  the  object-glass 
grows  larger;  so  it  is  easy  to  see  why  a  large  telescope  will 
divide  a  close  double  star  when  a  small  one  will  not:  the 
star  images,  which  are  of  sensible  diameter  and  consequently 
overlap,  as  seen  in  the  small  telescope,  are  reduced  by  the 
high  resolving  power  of  the  large  telescope  to  such  minute 
dimensions  that  they  appear  distinct  and  separate. 

Here,  perhaps,  a  word  of  explanation  may  be  useful;  for 
it  is  not  at  first  sight  obvious  that  a  star  should  appear 
smaller  in  a  large  telescope  than  in  a  small  one.  Such  a 
statement  would  not  be  true  of  the  Sun,  Moon,  or  planets. 
These  objects  are  all  comparatively  near  the  Earth,  and  even 
a  moderate  magnifying  power  will  show  them  (except  the 
most  distant  planets)  as  disks  on  which  structural  details  are 
visible.  The  stars,  however,  are  so  inconceivably  remote 
that  no  telescope,  however  powerful,  can  show  their  true 
disks.  They  are  mere  points  of  light,  brighter,  and  for  this 
reason  apparently  larger,  in  the  case  of  the  brilliant  stars,  but 
always  becoming  more  minute  and  pointlike  under  the  most 
favorable  atmospheric  conditions  and  with  the  most  powerful 
instruments. 

The  spurious  disks,  which  would  have  no  existence  if 


24  STELLAR  EVOLUTION 

light-waves  were  infinitely  short,  appear  large  in  small  tele- 
scopes, but  small  in  large  ones.  In  the  Yerkes  telescope,  for 
example,  stars  separated  by  only  a  tenth  of  a  second  of  arc 
can  be  resolved  under  the  best  atmospheric  conditions.  A 
four-inch  telescope  cannot  separate  stars  that  are  less  than  a 
second  of  arc  apart,  no  matter  what  magnifying  power  be 
applied.  In  such  an  instrument,  therefore,  the  thousands  of 
double  stars  whose  components  are  separated  by  less  than  a 
second  appear  as  single  stars.  In  the  same  way,  minute 
markings,  lying  close  together  on  the  Sun,  Moon,  or  planets, 
are  not  separately  distinguished  in  a  small  telescope,  while 
in  a  large  one  they  may  be  seen  as  distinct  objects,  provided 
the  atmospheric  conditions  are  sufficiently  favorable. 

We  may  sum  up  the  preceding  remarks  by  saying  that  in 
all  astronomical  observations  which  involve  the  separate  and 
distinct  recognition  of  very  closely  adjacent  stars,  or  a  knowl- 
edge of  the  most  minute  structure  of  the  Sun,  Moon,  or 
planets,  large  telescopes  must  be  employed  under  excellent 
atmospheric  conditions.  Furthermore,  if  it  is  a  question  of 
collecting  sufficient  light,  either  for  eye  observations,  or  for 
photography,  or  for  spectroscopic  analysis,  from  an  extremely 
faint  star,  the  great  area  of  a  large  object-glass  or  mirror  also 
becomes  essential.  Nevertheless  it  will  be  shown  that  for 
many  important  investigations  small  telescopes  are  equal  or 
even  superior  to  large  ones. 

This  brings  us  to  the  much-discussed  question  of  the 
relative  advantages  of  large  and  small  telescopes,  regarding 
which  a  great  deal  has  been  written.  On  the  one  hand,  we 
hear  the  amusing  claims  of  the  promoters  of  the  great  tele- 
scope which  was  to  be  the  clou  of  the  last  Paris  Exposition. 
This  immense  instrument — which  does  not  seem  to  have  been 
completed,  and  is  now  lying  unused — was  to  bring  the  Moon 
within  the  observer's  grasp — if  he  could  reach  a  meter! 
The  light-heartedness  of  this  claim  is  manifest  when  it  is 


LARGE  AND  SMALL  TELESCOPES       25 

remembered  that  no  existing  telescope,  under  the  best 
atmospheric  conditions,  has  ever  shown  the  Moon  as  well  as 
it  would  appear  to  the  unaided  eye  at  a  distance  of  fifty 
miles. 

On  the  other  hand,  it  has  been  stated,  with  great  insist- 
ence, that  it  is  absurd  to  use  a  telescope  of  more  than  four 
inches'  aperture  east  of  the  Mississippi  River,  or  of  more  than 
six  inches'  aperture  in  the  better  atmospheric  conditions 
west  of  it.  This  statement,  although  not  so  extreme  as  the 
one  which  emanated  from  Paris,  is  entirely  misleading  and 
unwarranted  by  the  facts.  It  was  probably  intended  to 
emphasize  a  conviction  that  the  atmospheric  conditions  in 
the  eastern  part  of  the  United  States  are  very  bad,  and  un- 
suited  for  large  telescopes.  Now,  it  is  quite  true  that  atmos- 
pheric disturbances  are  the  bane  of  astronomers  in  all  parts 
of  the  world ;  we  shall  have  occasion  to  discuss  this  question 
in  a  future  chapter.  It  is  also  true  that  the  meteorological 
conditions  are,  on  the  average,  much  more  favorable  for 
astronomical  observations  in  the  southwestern  part  of  the 
United  States  than  east  of  the  Mississippi  River.  But  it 
cannot  be  denied  that  many  of  the  valuable  observations 
turned  out  by  our  eastern  observatories  are  directly  due  to 
the  fact  that  they  are  equipped  with  large  telescopes.  That 
these  telescopes  would  do  more  and  better  work  under  better 
conditions  goes  without  saying.  Most  of  them  would  not 
exist  at  all,  however,  if  it  had  been  a  question  of  establishing 
them  some  thousands  of  miles  from  the  universities  or  col- 
leges with  which  they  are  connected. 

To  those  who  have  used  both  large  and  small  telescopes, 
the  great  advantages  of  large  instruments  for  many  kinds  of 
work  are  well  known.  I  have  heard  a  European  astronomer 
exclaim,  when  looking  at  Jupiter  for  the  first  time  with  the 
forty-inch  Yerkes  telescope,  that  his  years  of  study  of  this 
planet  with  a  small  telescope  seemed  almost  useless,  so  much 


26  STELLAR  EVOLUTION 

more  of  detail  could  he  perceive  at  a  single  glance.  I  have 
seen  minute  structure  on  the  Moon  with  this  telescope,  no 
trace  of  which  could  be  made  out  with  a  twelve-inch  tele- 
scope on  the  same  evening.  Countless  fine  bright  lines  in 
the  spectrum  of  the  chromosphere,  which  could  never  be 
detected  with  the  twelve-inch,  are  easily  seen  with  the 
forty-inch.  Burnham  has  separated  double  stars  at  the 
theoretical  limit  of  resolution  of  the  Yerkes  telescope,  and 
Barnard  has  observed  the  tiny  fifth  satellite  of  Jupiter  when 
it  was  beyond  the  reach  of  all  but  the  largest  existing  instru- 
ments. When  I  think  of  these  observations  and  of  Ritchey's 
photographs  of  the  Moon  and  star  clusters,  Frost's  and 
Adams'  photographs  of  the  spectra  of  faint  stars,  and  the  no 
less  important  results  obtained  by  Schlesinger,  Parkhurst, 
Ellerman,  Fox,  and  others  with  the  Yerkes  telescope;  and 
when  I  remember  that  most  of  these  observations  or  results 
could  not  have  been  obtained  with  a  small  telescope,  I  see  no 
possible  reason  for  denying  the  manifold  advantages  of  large 
instruments.  My  illustrations  have  been  confined  to  obser- 
vations made  with  the  Yerkes  telescope,  because  of  personal 
knowledge  of  them.  But  they  could  be  greatly  multiplied  if 
the  remarkable  work  of  the  Lick  telescope  and  of  other  large 
instruments  were  drawn  upon  for  examples.  In  the  next 
chapter,  through  the  aid  of  photography,  some  of  the  relative 
advantages  of  large  and  small  telescopes  will  be  illustrated. 


CHAPTER  V 

ASTRONOMICAL  PHOTOGRAPHY  WITH  CAMERA  LENSES 

THE  emphasis  laid  in  the  last  chapter  on  the  importance 
of  large  telescopes  must  not  be  supposed  to  mean  that  small 
telescopes  are  of  little  value.  The  single  fact  that  Burnham 
discovered  451  new  double  stars  with  a  six-inch  refractor 
(Plate  V)  is  sufficient  evidence  to  the  contrary.  It  is  quite 
true  that  small  telescopes  are  not  well  adapted  for  certain 
classes  of  work,  in  which  large  telescopes  excel.  But  their 
superiority  over  large  telescopes  is  no  less  evident  in  other 
fields.  The  equipment  of  an  observatory  recognizes  this  by 
the  provision  of  both  large  and  small  telescopes,  each  designed 
for  use  in  the  investigations  for  which  it  is  particularly  suited. 
In  fact,  the  characteristic  of  a  modern  astrophysical  observa- 
tory which  distinguishes  it  most  clearly  from  the  old  observa- 
tory of  one  or  two  instruments  is  the  careful  adaptation  of  a 
multiplicity  of  special  apparatus  to  certain  narrowly  defined 
purposes.  The  day  of  the  universal  instrument  has  passed, 
for  conditions  similar  to  those  which  have  resulted  in  the 
development  of  the  innumerable  special  tools  of  the  modern 
machine  shop  obtain  also  in  the  observatory. 

The  amateur  astronomer  should  keep  this  fact  clearly  in 
mind.  There  is  some  reason  to  fear  that  the  large  and 
expensive  equipments  of  modern  observatories  have  tended 
to  discourage  the  worker  with  small  instruments.  As  one 
who  has  looked  at  the  subject  from  both  sides,  I  may  be 
permitted  to  oppose  this  pessimistic  view.  Far  from  believ- 
ing that  recent  developments  have  been  detrimental  to  the 
amateur,  I  am  strongly  of  the  opinion  that  his  opportunities 
for  useful  work  have  never  before  been  so  numerous.  The 

27 


28  STELLAK  EVOLUTION 

importance  of  this  subject,  due  to  the  high  value  of  the 
contributions  to  astronomy  made  by  amateurs  in  the  past, 
has  led  me  to  devote  a  subsequent  chapter  to  opportunities 
for  work  with  inexpensive  instruments. 

In  considering  the  peculiar  advantages  of  small  telescopes 
in  certain  fields  of  research,  attention  must  be  called  at  the 
outset  to  the  important  part  played  by  photography  in  the 
astrophysical  work  of  the  present  day.  The  photographic 
plate,  through  its  power  of  storing  up  impressions  made  by 
feebly  luminous  rays,  has  in  most  cases  an  immense  advan- 
tage over  the  eye.  The  eye  perceives  almost  at  once  as  much 
as  can  be  seen  by  long  gazing  at  a  faint  object.  But  the 
photographic  plate  continues,  hour  after  hour,  and  perhaps 
night  after  night,  to  accumulate  impressions,  so  that  with 
sufficiently  long  exposures,  objects  far  too  faint  to  be  seen 
by  the  eye  with  the  same  telescope  are  clearly  and  per- 
manently recorded.  Moreover,  the  photographic  plate  is 
sensitive  to  light-waves  which  are  too  short  to  produce  the 
least  effect  upon  the  eye,  and  in  this  power  of  recording 
objects  which  otherwise  could  never  be  rendered  visible,  no 
matter  what  their  intensity  of  radiation,  the  plate  presents  a 
second  great  advantage.  Because  of  these  and  other  points 
of  superiority,  which  far  outweigh  certain  slight  defects  that 
in  some  few  instances  still  leave  the  plate  inferior  to  the  eye, 
the  photographic  method  is  now  exclusively  employed  for 
many  kinds  of  observations. 

Some  of  the  most  important  results  of  astronomy  have 
been  derived  from  the  use  of  an  ordinary  camera,  having 
just  such  a  lens  as  is  found  in  the  possession  of  thousands 
of  amateur  photographers.  If  we  take  an  ordinary  camera 
and  point  it  on  a  clear  night  toward  the  north  pole,  it  will 
be  found,  after  an  exposure  of  one  or  two  hours,  that  the 
stars  which  surround  the  pole  have  drawn  arcs  of  circles 
upon  the  plate  (Plate  VI).  This  is  due  to  the  fact  that 


ASTRONOMICAL  PHOTOGRAPHY       29 

the  Earth  is  rotating  upon  its  axis  at  such  a  rate  as  to  cause 
every  star  in  the  sky  to  appear  to  travel  through  a  complete 
circle  once  in  twenty-four  hours.  Since  the  pole  is  -the  place 
in  the  sky  toward  which  the  Earth's  axis  is  pointing,  it  is 
easy  to  understand  that  the  nearer  the  star  lies  to  the  pole, 
the  smaller  does  this  circle  become.  As  we  move  away  from 
the  pole  we  find  the  curvature  of  the  star  trails  growing  less 
and  less,  until  at  the  equator  they  appear  as  straight  lines. 

Just  such  photographs  as  these  are  frequently  employed 
in  astrophysical  investigations;  e.  g.,  for  the  purpose  of 
recording  variations  in  a  star's  brightness,  which  would  be 
shown  on  the  plate  by  changes  in  the  strength  of  the  trail. 
But  for  most  purposes  it  is  desirable  to  have  photographs  of 
stars  in  which  they  are  represented  as  points  of  light  rather 
than  as  lines.  To  obtain  these  photographs  it  is  necessary  to 
mount  the  camera  in  such  a  way  that  it  can  be  turned  about 
an  axis  parallel  to  the  Earth's  axis,  at  a  perfectly  uniform 
rate,  once  in  twenty-four  hours.  A  camera  so  mounted 
becomes  an  equatorial  photographic  telescope,  differing  in  no 
important  respect,  save  in  the  construction  of  its  lens,  from 
the  largest  refractors. 

Here,  for  example,  is  a  photograph  (Plate  VII)  of  the 
Bruce  photographic  telescope  of  the  Yerkes  Observatory. 
This  instrument  has  a  compound  lens  ten  inches  in  diameter, 
made  by  Brashear  from  four  lenses  suitably  combined,  of 
such  curvature  as  to  form  an  image  at  a  point  only  fifty 
inches  distant  from  the  optical  center  of  the  lens  system.  It 
will  be  seen  that  such  a  lens  must  produce  a  very  bright  and 
highly  concentrated  image,  in  which  the  various  objects  are 
Crowded  close  together  because  of  the  small  scale  of  the 
picture.  If  the  same  lens  were  so  constructed  as  to  form  an 
image  ten  times  as  far  distant  from  the  photographic  plate, 
the  several  elements  of  the  picture  would  then  be  ten  times 
more  widely  separated,  and  a  longer  time  would  be  required 


30  STELLAR  EVOLUTION 

to  photograph  them,  on  account  of  the  spreading  of  the  same 
amount  of  light  over  a  larger  surface.  As  will  be  seen  from 
the  illustration,  the  tube  which  carries  the  lens  and  photo- 
graphic plate  is  mounted  in  such  a  way  that  it  may  be  turned 
about  an  axis  parallel  to  the  axis  of  the  Earth  by  means  of  a 
driving-clock,  placed  in  the  upper  part  of  the  iron  supporting 
column.  The  same  mounting  carries  not  only  the  ten-inch 
lens,  but  also  the  lens  of  a  guiding  telescope,  through  which 
the  observer  watches  a  star  during  the  entire  period  of 
exposure,  continued,  perhaps,  for  many  hours.  He  may 
thus  correct  any  slight  irregularity  in  the  motion  of  the  tele- 
scope by  certain  screws  provided  for  the  purpose,  which  per- 
mit him  to  keep  the  star  accurately  at  the  intersection  of  two 
illuminated  cross-wires.  The  driving  of  the  clock  is  so  accu- 
rate that  this  is  accomplished  almost  automatically,  though 
small  changes  in  atmospheric  refraction  and  other  causes 
require  minute  displacements  of  the  instrument  to  be  made 
from  time  to  time,  to  insure  the  perfect  immobility  of  the 
stellar  images  upon  the  photographic  plate. 

Besides  the  ten-inch  camera  and  the  guiding  telescope, 
the  Bruce  telescope  carries  three  other  cameras,  with  lenses 
of  6  inches,  3.4  inches,  and  1.6  inches  aperture  respectively. 
Thus  four  photographs  of  the  same  part  of  the  heavens,  on 
different  scales,  determined  by  the  focal  lengths  of  the 
lenses,  are  obtained  in  a  single  operation.  Our  knowledge 
of  the  structure  of  the  vast  girdle  of  stars  that  forms  the 
Milky  Way  is  derived  in  very  large  part  from  a  study  of 
photographs  made  with  such  an  instrument.  At  the  Lick 
Observatory  Barnard  used  the  six-inch  Willard  lens  to  great 
advantage  in  photographing  these  star  clouds,  and  of  late, 
through  the  opportunity  afforded  by  the  Hooker  Expedition 
at  the  lower  latitude  of  Mount  Wilson,  he  has  carried  his 
work  farther  south  of  the  celestial  equator.  The  Bruce  tele- 
scope, temporarily  transferred  from  the  Yerkes  Observatory  to 


ASTRONOMICAL  PHOTOGRAPHY        31 

Mount  Wilson  for  use  during  the  spring  and  summer  of  1905, 
has  yielded  some  remarkably  fine  results  in  Barnard's  hands. 
The  smallest  of  the  four  photographs  made  in  a  single  opera- 
tion is  taken  with  an  ordinary  "magic-lantern"  lens  of  1.6 
inches  aperture  and  6.4  inches  focal  length.  This  shows  a 
a  region  about  fifteen  degrees1  across  within  a  circular  area 
about  1.7  inches  in  diameter  on  the  photographic  plate. 
With  the  ten-inch  lens  the  field  of  sharply  defined  images  is 
limited  to  about  eight  degrees,  but  it  is  still  large  enough 
to  include  extensive  star  clouds  and  nebulae.  The  larger 
scale,  due  to  the  greater  focal  length  of  the  ten-inch  lens, 
brings  out  details  of  structure  that  are  not  visible  on  the 
smaller  photographs.  Plates  VIII  and  IX,  reduced  from  the 
originals  in  the  same  proportion,  illustrate  the  relative  scales 
of  the  photographs  made  with  the  two  lenses. 

The  Milky  Way,  as  revealed  by  such  photographs,  is 
a  most  extraordinary  spectacle.  The  countless  stars  that 
compose  it  are  grouped  in  every  conceivable  manner,  and 
intertwined  with  long  reaches  of  diffuse  nebulous  clouds. 
Here  and  there  vacant  regions,  sometimes  apparently  darker 
than  the  background  of  the  heavens,  resemble  vast  lanes, 
extending  through  the  entire  thickness  of  the  star  clouds,  or 
perhaps  lead  one  to  suspect  that  an  obscuring  medium  may 
be  cutting  off  the  light  from  immeasurably  distant  bodies. 
Again,  a  nebula  of  great  extent,  diffuse  on  one  side  and  sharply 
bounded  on  the  other,  may  suggest  the  action  of  forces  be- 
yond our  present  means  of  investigation.  The  filmy  veils 
spread  by  certain  nebulae  seem  to  envelop  the  stars  in  mist, 
though  in  most  cases  we  cannot  say  with  certainty  whether 
the  stars  are  actually  within  the  clouds,  or  remote  from  them 
in  the  line  of  vision.  The  surest  test  of  relationship  between 
stars  and  surrounding  nebulae  is  afforded  by  the  spectroscope, 

1  Readers  who  are  not  accustomed  to  angular  measure  may  be  reminded  that 
the  two  "pointers"  of  the  "  Dipper  "  are  about  five  degrees  apart. 


32  STELLAK  EVOLUTION 

as  will  be  shown  in  a  subsequent  chapter.  It  has  been  found 
that  stars  of  different  spectral  types,  which  are  ordinarily 
assumed  to  indicate  different  degrees  of  development,  are 
not  equally  represented  in  the  Milky  Way.  The  connection 
between  these  stars  and  surrounding  nebulae,  and  the  possible 
relationship  between  spectral  type  and  the  grouping  of  the 
stars  in  the  cloudlike  forms  of  the  Galaxy,  is  one  of  the 
important  problems  of  the  present  time.  Our  knowledge  of 
the  Milky  Way  and  its  structure  is  still  very  meager,  but  the 
future  is  certain  to  bring  great  advances. 

These  illustrations  may  suffice  to  show  the  uses  of  the 
ordinary  camera  lens  in  investigations  bearing  upon  the 
general  structure  of  the  Milky  Way.  A  simple  compari- 
son will  serve  to  bring  out  both  the  advantages  and  dis- 
advantages of  large  telescopes  in  studies  of  a  similar  kind. 
Plate  X  shows  the  Milky  Way  in  Ophiucus  from  one  of  Bar- 
nard's photographs  made  with  a  portrait  lens.  It  affords  a 
superb  picture  of  this  part  of  the  sky,  such  as  no  visual 
observations  with  any  telescope  could  supply.  If  the  same 
region  of  the  heavens  were  examined  with  a  large  telescope, 
the  field  of  view  would  be  so  restricted  that  no  proper 
impression  could  be  obtained  as  to  the  character  of  the 
Milky  Way  or  the  distribution  of  the  stars  within  it.  It 
would,  of  course,  be  possible  to  count  one  by  one  the  hun- 
dreds of  stars  included  within  a  single  field  of  view,  and  by 
long  and  laborious  measurements  to  map  these  stars  and 
ultimately  to  build  up,  from  combination  into  a  single  picture 
of  the  results  thus  obtained,  a  representation  of  the  Milky 
Way.  However,  such  a  task  would  occupy  years  of  labor, 
and  the  result  would  be  less  valuable,  for  many  purposes, 
than  that  illustrated  in  Plate  X.  This  picture  is  an  auto- 
graphic record,  showing  not  only  the  distribution  of  the 
stars,  but  also  their  relative  brightness  on  the  date  of  the 
exposure. 


ASTRONOMICAL  PHOTOGRAPHY       33 

Since  such  results  are  due  to  photography,  the  comparative 
value  of  large  telescopes  should  be  judged  by  the  same 
means.  Plate  XI  is  a  reproduction  of  a  photograph  of  the 
cluster  Messier  11,  which  is  represented  in  Plate  VIII  as  a 
small  circular  white  dot  in  the  upper  part  of  the  picture. 
The  short  focal  length  of  the  camera  lens,  which  causes  it  to 
form  upon  the  plate  a  small-scale  picture  covering  a  large 
region  in  the  heavens,  is  not  competent  to  separate  out  the 
single  stars  of  this  cluster.  The  photograph  reproduced  in 
Plate  XI  was  made  by  Ritchey  with  the  forty-inch  Yerkes 
telescope,  which  has  a  focal  length  of  sixty-four  feet,  as  com- 
pared with  the  focal  length  of  6.4  inches  of  the  camera  lens 
used  for  Barnard's  photograph.  The  scale  of  the  negative 
obtained  with  the  Yerkes  telescope  is  therefore  about  120 
times  as  great  as  in  the  case  of  the  camera  lens.  This 
great  scale,  while  disadvantageous  so  far  as  it  bears  upon 
the  question  of  the  general  structure  of  the  Milky  Way, 
would  be  in  the  highest  degree  advantageous  if  the  problem 
under  consideration  involved  the  study  of  the  individual  stars 
in  the  cluster  Messier  11.  With  the  camera  lens  these  stars 
are  so  close  together  upon  the  plate  that  their  separate  images 
are  confused.  With  the  Yerkes  telescope  the  images  are 
widely  separated  from  one  another,  permitting  the  position 
and  the  brightness  of  each  star  to  be  determined  with  great, 
precision.  The  Bruce  lens  gives  an  intermediate  scale.  If 
Plate  IX  had  been  enlarged  in  the  same  proportion  as  Plate 
XI,  this  cluster  would  be  shown  fairly  well  resolved.  But 
Messier  13  (Plate  XIX)  is  far  beyond  the  capacity  of  the 
Bruce  lens. 

It  may  be  of  interest  to  include  here  another  photograph 
illustrative  of  the  advantages  of  great  focal  length  for  certain 
classes  of  work.  Plate  XII  represents  a  photograph  of  the 
Moon,  made  by  Ritchey  with  the  twelve-inch  Kenwood  tele- 
scope, which  is  eighteen  feet  long.  This  picture  gives  an 


34  STELLAR  EVOLUTION 

excellent  general  idea  of  the  lunar  topography.  But  if  the 
detailed  structure  of  the  lunar  mountains  is  to  be  investigated, 
such  a  picture  as  that  reproduced  in  Plate  XIII  would  evi- 
dently be  far  more  effective  for  the  purpose.  Theophilus,  the 
great  ring  mountain  here  represented,  may  be  seen  in  Plate 
XII  on  a  smaller  scale.  The  large-scale  picture  was  obtained 
by  Bitch ey  with  the  forty-inch  telescope,  which,  as  already 
remarked,  has  a  focal  length  of  sixty-four  feet.  The  scale 
of  the  original  photograph  was  therefore  about  three  and 
one-half  times  as  great  as  that  of  the  photograph  taken  with 
the  Kenwood  telescope.  In  consequence  of  the  larger  scale 
of  the  Yerkes  picture,  it  brings  out  many  small  details  which 
are  entirely  lacking  on  the  Kenwood  photograph. 

These  illustrations  of  the  separating  power  of  the  large 
telescope  may  lead  us  to  an  examination  of  the  instrument 
itself  (Plate  XIV) .  Although  so  much  larger,  it  differs  in  no 
essential  particulars  from  the  Bruce  photographic  telescope, 
also  made  by  the  firm  of  Warner  &  Swasey.  The  great 
weight  of  the  forty-inch  lens,  amounting  with  its  cell  to  half 
a  ton,  requires  that  the  tube  which  carries  it  shall  be  of 
immense  rigidity  and  strength.  This  tube,  sixty-four  feet 
in  length,  is  supported  at  its  middle  point  by  the  declination 
axis,  which  in  its  turn  is  carried  by  the  polar  axis,  adjusted 
to  accurate  parallelism  with  the  axis  of  the  Earth.  By 
means  of  driving  mechanism  in  the  upper  section  of  the  iron 
column,  the  whole  instrument  is  turned  about  this  polar 
axis  at  such  a  rate  that  it  would  complete  one  revolution  in 
twenty-four  hours.  Although  the  moving  parts  weigh  over 
twenty  tons,  the  telescope  can  be  directed  to  any  part  of 
the  sky  by  hand,  but  this  operation  is  much  facilitated  by 
the  use  of  electric  motors  provided  for  the  purpose.  When 
once  directed  toward  the  object  to  be  observed,  it  will  fre- 
quently happen  that  the  lower  end  of  the  telescope  is  far 
out  of  reach  above  the  observer's  head.  For  this  reason  the 


ASTRONOMICAL  PHOTOGRAPHY       35 

entire  floor  of  the  observing-room,  seventy-five  feet  in  diam- 
eter, is  constructed  like  an  electric  elevator,  which,  by  moving 
a  lever,  can  be  made  to  rise  or  fall  through  a  distance  of 
twenty-three  feet.  Thus  the  lower  end  of  the  telescope  is 
rendered  accessible  even  when  the  object  is  near  the  horizon 
(Plate  XV).  In  order  that  the  observing  slit  maybe  di- 
rected to  any  part  of  the  sky,  the  dome,  ninety  feet  in  diameter 
(Plate  XVI),  is  mounted  on  wheels  and  can  be  turned  to  any 
desired  position  by  means  of  an  electric  motor  controlled 
from  the  rising-floor. 

The  telescope  is  used  for  a  great  variety  of  purposes  in 
conjunction  with  appropriate  instruments,  which  are  attached 
to  the  lower  end  of  the  tube  near  the  point  where  the  image 
is  formed.  We  have  already  examined  a  photograph  of  a  star 
cluster  taken  with  this  telescope,  but  without  describing  the 
process  of  making  it.  As  a  matter  of  fact,  the  forty-inch 
object-glass  was  designed  for  visual  observations,  and  its 
maker,  the  late  Alvan  Gr.  Clark,  had  no  idea  that  it  would  ever 
be  employed  for  photography.  Without  dwelling  upon  the 
distinguishing  features  of  visual  and  photographic  lenses,  it 
may  be  said  that  the  former  are  so  designed  by  the  optician 
as  to  unite  into  an  image  those  rays  of  light,  particularly  the 
yellow  and  the  green,  to  which  the  eye  is  most  sensitive. 
With  the  only  varieties  of  optical  glass  obtainable  in  large 
pieces,  it  is  impossible  to  unite  into  a  single  clearly  defined 
image  all  of  the  red,  the  yellow,  the  green,  the  blue,  and  the 
violet  rays  that  reach  us  from  a  star.  Therefore,  when  the 
optician  decides  to  produce  an  image  most  suitable  for  eye 
observations,  he  deliberately  discards  the  blue  and  violet 
rays,  simply  because  they  are  less  important  to  the  eye  than 
the  yellow  and  green  rays.  For  this  reason  the  image  of  a 
star  produced  by  a  large  visual  refracting  telescope  is  sur- 
rounded by  a  blue  halo,  containing  the  rays  discarded  by 
the  optician.  These  very  rays,  however,  are  the  ones  to  which 


36  STELLAR  EVOLUTION 

the  ordinary  photographic  plate  is  most  sensitive;  hence  in 
a  photographic  telescope  the  blue  and  violet  rays  are  united, 
while  the  yellow  and  green  rays  are  discarded. 

The  forty -inch  telescope  is  of  the  first  type,  constructed 
primarily  for  visual  observations.  In  order  to  adapt  it  for 
photography,  Ritchey,  then  a  member  of  the  Yerkes  Obser- 
vatory staff,  simply  placed  before  the  (isochromatic)  plate  a 
thin  screen  of  yellow  glass,  which  cuts  out  the  blue  rays,  but 
allows  the  yellow  and  green  rays  to  pass.  As  isochromatic 
plates  are  sensitive  to  yellow  and  green  light,  there  is  no 
difficulty  in  securing  an  image  with  the  rays  which  the 
object-glass  unites  into  a  perfect  image.  During  the  entire 
time  of  the  exposure  some  star  lying  just  outside  the  region 
to  be  photographed  is  observed  through  an  eye-piece  mag- 
nifying 1,000  diameters.  This  eye-piece  is  attached  to  the 
frame  which  carries  the  photographic  plate,  and  is  suscep- 
tible of  motion  in  two  directions  at  right  angles  to  one  an- 
other (Plate  XVII).  In  the  center  of  the  eye-piece  are  two 
very  fine  cross-lines  of  spider  web,  illuminated  by  a  small 
incandescent  lamp.  If  the  observer  notices  that  through 
some  slight  irregularity  in  the  motion  of  the  telescope,  or 
some  change  of  refraction  in  the  Earth's  atmosphere,  the 
star  image  is  moving  away  from  the  point  of  intersection 
of  the  cross-lines,  he  instantly  brings  it  back  by  one  or  both 
of  the  screws.  As  the  plate  moves  with  the  eye-piece,  it  is 
evident  that  this  method  furnishes  a  means  of  keeping  the 
star  images  exactly  at  the  same  position  on  the  plate  through- 
out the  entire  exposure. 

Many  other  comparisons  of  large  and  small  telescopes 
might  be  given,  and  some  of  these  will  be  included  in  sub- 
sequent chapters.  They  all  serve  to  demonstrate  that  each 
telescope  has  advantages  and  disadvantages  peculiar  to  its 
size  and  type  of  construction.  For  some  purposes  small 
camera  lenses  are  to  be  preferred  to  all  other  instruments. 


ASTRONOMICAL  PHOTOGRAPHY       37 

In  fact,  without  their  aid  many  investigations  of  the  highest 
importance  could  never  be  undertaken.  For  other  investiga- 
tions these  short-focus  instruments  may  be  entirely  unsuited, 
while  refracting  telescopes  of  great  focal  length  may  give 
excellent  results.  These  larger  telescopes  also  have  their 
limitations,  and  must  yield  to  reflecting  telescopes  in  certain 
other  kinds  of  work.  The  truth  of  this  statement  will  be 
brought  out  in  the  next  chapter. 


CHAPTER  VI 

DEVELOPMENT  OF  THE  REFLECTING  TELESCOPE 

ON  a  night  in  April,  1845,  while  sweeping  the  sky  in 
the  constellation  of  the  Hunting  Dogs,  the  observers  with 
the  great  Parsonstown  reflector  discovered  a  spiral  nebula. 
The  instrument  with  which  the  discovery  was  made  may  well 
be  regarded  as  one  of  the  most  remarkable  scientific  achieve- 
ments of  the  nineteenth  century.  With  its  immense  mirror, 
six  feet  in  diameter,  having  a  focal  length  of  fifty-four  feet, 
the  great  telescope  of  Lord  Rosse  surpassed  in  size  all  others 
ever  constructed.  Unfortunately  for  the  progress  of  science, 
the  engineering  methods  of  that  day  were  inadequate  to 
provide  a  suitable  mounting  for  this  gigantic  instrument. 
All  parts  of  the  machinery  had  to  be  constructed  on  the  spot, 
with  such  tools  as  the  period  and  the  circumstances  afforded. 
It  is  no  small  credit  to  the  Earl  of  Rosse  that  under  these 
conditions  the  telescope  was  ever  erected,  and  kept  in  active 
use  by  an  able  company  of  observers.  Supported  upon  a  ball- 
and-socket  joint  at  its  lower  end,  the  enormous  tube,  swung 
in  chains,  was  confined  to  observations  within  a  short  distance 
of  the  meridian  by  two  flanking  stone  walls.  The  observer, 
mounted  upon  a  platform  far  above  the  ground,  saw  the 
image  of  an  object  as  he  looked  down  into  the  tube.  To 
the  present-day  astronomer,  provided  with  every  appliance 
to  facilitate  the  finding  of  an  object,  and  with  an  accurate 
driving-clock  which  moves  the  telescope  so  steadily  and  uni- 
formly as  to  maintain  the  image  in  the  field  of  view  for  hours 
together,  it  is  little  short  of  marvelous  that  the  observers 
with  the  great  Parsonstown  reflector  were  able  to  obtain 
results  of  value.  But,  in  spite  of  the  difficulties  to  be  over- 

38 


DEVELOPMENT  OF  KEFLECTING  TELESCOPE   39 

come,  both  in  manipulating  the  telescope  and  in  finding 
opportunities  for  observation  under  the  cloudy  skies  of 
Ireland,  Lord  Eosse  and  his  assistants  recorded  many  val- 
uable discoveries  in  their  memoirs.  Of  all  these  discoveries 
that  of  the  spiral  nebula  in  Canes  Venatici  was  perhaps 
the  most  significant  of  the  future  (Plate  LXXXVIII).  Be- 
fore this  chapter  is  concluded  we  shall  see  how  this  beautiful 
object,  which  once  stood  alone  among  the  heavenly  bodies 
as  the  only  visible  representative  of  a  distinctly  spiral  form, 
has  now  come  to  be  regarded,  through  the  work  of  Keeler,  as 
a  type  of  the  most  interesting  and  the  most  numerous  class 
of  nebulae. 

The  history  of  the  Parsonstown  reflector  has  in  some  de- 
gree resembled  that  of  almost  every  reflecting  telescope  ever 
built.  The  infinite  care  expended  by  Herschel  and  by  others 
who  have  followed  him  in  the  construction  of  mirrors  for  such 
instruments  has  been  in  large  part  annulled  by  the  imper- 
fections of  the  mountings  provided  for  the  mirrors.  In 
the  period  that  preceded  the  introduction  of  photographic 
methods,  these  imperfections  were  far  less  serious  than  they 
would  be  considered  from  our  present  point  of  view.  It  is 
true  that  they  hampered  observation,  and  in  the  early  days 
rendered  accurate  measurement  with  the  telescope  practically 
impossible.  But  the  employment  of  the  photographic  plate 
has  imposed  a  new  condition,  rigorous  and  unyielding,  upon 
the  constructors  of  telescope  mountings.  In  order  to  secure 
satisfactory  photographs,  which  shall  do  full  justice  to  the 
optical  qualities  of  the  instrument,  and  show  only  such  de- 
fects as  atmospheric  disturbances  may  produce,  it  is  necessary 
that  the  mirrors  be  so  rigidly  supported,  and  so  accurately 
moved  by  the  driving-clock,  that  a  stellar  image  shall  not 
depart,  during  exposures  of  many  hours,  by  so  much  as 
one-thousandth  of  an  inch  from  a  fixed  position  upon  the 
photographic  plate. 


4:0  STELLAR  EVOLUTION 

In  view  of  the  difficulties  to  be  overcome,  it  will  be  under- 
stood that  to  accomplish  such  a  result  is  no  small  task.  In 
the  first  place,  the  mirror,  which  is  so  sensitive  to  deformation 
that  it  will  bend  under  its  own  weight  unless  supported  by 
special  apparatus,  must  be  firmly  mounted,  yet  without  strain, 
at  the  lower  end  of  an  open  tube.  In  the  second  place,  pro- 
tection must  be  provided  against  currents  of  warm  and 
cold  air,  and  even  against  the  heat  radiated  from  the 
observer's  body,  on  account  of  the  great  sensitiveness  of  the 
mirror  to  heat,  and  of  the  light-rays  to  irregular  refraction  in 
the  telescope  tube.  These  precautions  having  been  taken, 
the  tube  must  be  so  mounted  that  it  can  be  moved  with  per- 
fect steadiness  and  uniformity  about  an  axis  parallel  to  the 
axis  of  the  Earth.  This  condition  is  imposed  by  the  neces- 
sity of  counteracting  the  apparent  motion  of  the  star  through 
the  heavens,  due  to  the  rotation  of  the  Earth.  But  while 
this  rotation  is  uniform,  the  motion  of  the  star  is  not,  since 
the  displacement  of  its  apparent  position  from  its  true  posi- 
tion in  the  heavens,  due  to  the  bending  of  its  rays  during 
transmission  through  the  Earth's  atmosphere,  varies  with  the 
height  of  the  star  above  the  horizon.  It  therefore  becomes 
necessary,  as  previously  explained,  to  supplement  the  uniform 
motion  of  the  driving-clock  by  corrections,  accomplished  by 
the  hands  of  an  observer.  All  these  obstacles  having  been 
surmounted,  there  still  remain  serious  sources  of  difficulty 
in  the  shaking  of  the  telescope  by  the  wind,  the  changes  of 
temperature  during  the  exposure,  which  alter  the  focal  length 
of  the  mirror,  and  finally,  most  serious  of  all,  disturbances 
in  the  atmosphere  which  tend  to  blur  and  confuse  the  image, 
instead  of  leaving  it,  sharp  and  well  defined,  to  make  its 
record  upon  the  photographic  plate.  It  should  also  be 
remembered  that  the  observer  must  be  prepared  to  hold  his 
eye  at  the  eye-piece,  and  correct  every  few  seconds  the  posi- 
tion of  the  plate,  throughout  exposures  lasting  several  hours, 


DEVELOPMENT  OF  REFLECTING  TELESCOPE   41 

in  an  open  dome  where  the  temperature  may  not  infrequently 
be  below  zero. 

After  the  erection  of  Lord  Rosse's  great  reflector,  the 
attention  of  opticians  was  confined  mainly  to  the  construction 
of  refracting  telescopes,  which  grew  rapidly  in  size,  reaching 
apertures  of  fifteen  inches  in  the  Harvard  refractor  (1845), 
thirty-six  inches  in  the  Lick  refractor  (1889) ,  and  forty  inches 
in  the  Yerkes  refractor  (1897).  In  these  instruments  care- 
ful attention  was  given  to  all  details  of  mechanical  construc- 
tion, and  the  Lick  and  Yerkes  telescopes  are  among  the  most 
successful  products  of  modern  engineering. 

The  development  of  the  reflecting  telescope  has  been  due 
mainly  to  amateurs,  whereas  refractors  have  been  made  by 
professional  opticians  and  mounted  by  experienced  engineers. 
To  the  inadequate  equipment  of  the  amateur's  workshop  may 
therefore  be  ascribed  many  of  the  deficiencies  in  the  mount- 
ings of  reflecting  telescopes.  In  some  cases,  however,  re- 
flectors of  large  aperture,  figured  and  mounted  by  professional 
opticians  and  engineers,  have  given  results  of  little  or  no 
value.  In  these  cases,  as  in  others,  it  appears  that  insufficient 
attention  was  paid  to  the  excessive  sensitiveness  of  large 
mirrors,  which  causes  them  to  require  much  more  careful 
treatment  than  is  amply  sufficient  to  yield  good  images  with 
a  lens. 

In  stellar  spectroscopic  work  good  results  were  obtained 
with  reflectors  by  Huggins  and  Draper  at  a  comparatively 
early  period,  but  it  was  not  until  the  last  years  of  the  nine- 
teenth century  that  such  telescopes  were  employed  with  any 
considerable  degree  of  success  for  the  photography  of  nebulae. 
The  first  photograph  of  a  nebula  was  obtained  with  a  refract- 
ing telescope  by  Draper  in  1881.  Photographs  of  the  Great 
Nebula  in  Andromeda,  made  by  Roberts  in  1886  with  a 
twenty-inch  reflector,  showed  for  the  first  time  the  truly 
spiral  form  of  this  remarkable  object,  and  thus  indicated 


42  STELLAR  EVOLUTION 

some  of  the  great  possibilities  of  investigating  nebular 
structure  with  instruments  of  this  type.  Briefly  speaking, 
their  superiority  to  refractors  lies  in  the  fact  that  the  light 
is  not  weakened  by  passage  through  glass,  but,  after  reflec- 
tion from  a  surface  of  pure  silver,  all  the  rays,  independently 
of  their  color,  are  united  in  a  common  focus.  With  a 
refractor  many  of  the  rays  are  completely  cut  off  during 
transmission  through  the  glass  of  the  lens,  which  is  as 
impervious  as  so  much  steel  to  the  very  short  waves  of  the 
ultra-violet  spectrum.  Furthermore,  a  lens  does  not  unite 
all  the  rays  of  different  colors  into  a  single  focus,  but  forms 
a  series  of  images,  corresponding  to  light  of  different  wave- 
lengths. In  order  to  get  a  sharp  photograph  with  a  refract- 
ing telescope,  it  is  therefore  necessary  to  discard  some  of 
these  rays,  in  the  manner  already  described  (p.  35).  The 
reflector,  on  the  contrary,  utilizes  all  of  the  light1 — an  advan- 
tage which  is  clearly  shown  by  the  results  obtained  with  this 
type  of  telescope. 

The  photographic  studies  of  nebulae  made  by  Keelerwith 
the  Crossley  reflector  of  the  Lick  Observatory,  mark  a  step 
of  the  greatest  importance  in  the  development  of  the  reflecting 
telescope.  The  mounting  of  this  instrument,  constructed  in 
England  some  years  previously,  and  presented  to  the  Lick 
Observatory  by  Mr.  Crossley,  was  very  poorly  adapted  to  carry 
the  excellent  mirror  of  three  feet  aperture.  But  through  the 
extraordinary  efforts  of  Keeler,  whose  severe  exertions  in 
carrying  out  this  work  hastened  his  death,  the  mounting 
was  so  strengthened  and  improved  as  to  permit  remarkable 
results  to  be  obtained.  In  other  hands,  even  after  these 
improvements  had  been  made,  it  is  doubtful  whether  such 
exquisite  photographs  would  have  resulted.  But,  after  many 
unsuccessful  efforts,  Keeler  learned  how  to  overcome  the 
difficulties  peculiar  to  the  instrument,  and  in  this  he  has  been 

1  Except  a  certain  percentage  lost  in  reflection. 


DEVELOPMENT  or  REFLECTING  TELESCOPE      43 

ably  followed  by  Perrine.1  We  shall  have  occasion  later  to 
refer  to  their  results. 

The  mounting  of  the  two-foot  reflecting  telescope  of  the 
Yerkes  Observatory  was  designed  expressly  for  photographic 
purposes,  and  no  pains  were  spared  to  adapt  the  instrument 
for  the  exacting  requirements  of  such  work.  The  mirror,  23^ 
inches  in  diameter  and  of  93  inches  focal  length,  was  con- 
structed by  Ritchey  in  1895  at  his  home  in  Chicago.  This 
mirror  is  of  the  highest  quality,  meeting  the  most  severe 
optical  tests  that  can  be  applied  to  it.  The  mounting  of  the 
telescope,  designed  by  Wadsworth,  with  modifications  by 
Ritchey,  was  constructed  in  the  instrument  shop  of  the  Yerkes 
Observatory,  and  is  very  heavy  and  rigid.  In  the  photograph 
(Plate  XVIII)  the  mirror  may  be  seen  in  position  at  the  lower 
end  of  the  skeleton  tube.  At  the  upper  end  of  this  tube  is 
a  small  plane  mirror,  so  supported  that  its  face  makes  an 
angle  of  45°  with  the  axis  of  the  tube.  The  telescope  is 
therefore  of  the  Newtonian  type,  the  image  being  formed  on 
the  photographic  plate  near  the  upper  end  of  the  tube,  after 
reflection  of  the  cone  of  rays  from  the  small  mirror.  The 
double  slide  plate-carrier,  which  holds  a  plate  3^  X  4J  inches 
in  size,  is  precisely  similar  to  the  plate-carrier  employed  with 
the  forty-inch  refractor  (Plate  XVII). 

A  comparison  of  the  results  obtained  with  this  instrument, 
with  those  secured  with  the  forty -inch  Yerkes  refractor,  will 
suffice  to  show  the  peculiar  advantages  of  the  reflector  for 
certain  kinds  of  work.  It  should  not  be  forgotten  that  the 
forty-inch  refractor  has  other  advantages,  which  fit  it  for 
work  that  could  not  be  done  under  any  circumstances  with 
the  two-foot  reflector.2  But  in  the  photography  of  faint 

1A  new  and  satisfactory  mounting  has  since  been  constructed  for  the  Crossley 
reflector. 

2  For  example,  the  scale  of  the  images  given  by  the  forty-inch  is  eight  times  that 
of  the  two-foot  reflector.  Moreover,  the  former  is  well  adapted  for  work  on  the  Sun, 
for  which  the  latter  cannot  be  used. 


44  STELLAR  EVOLUTION 

stars,  particularly  in  the  photography  of  nebulae,  the  two- 
foot  reflector  is  especially  useful.  It  is  possible  with  this 
instrument,  in  an  exposure  of  only  forty  minutes,  to  photo- 
graph stars  which  are  at  the  extreme  limit  of  vision  with  the 
forty-inch  refractor.  With  longer  exposures,  countless  stars, 
which  can  never  be  seen  or  photographed  with  the  large 
refractor,  are  recorded  on  the  plates.  Compare,  for  example, 
the  photographs  of  the  star  cluster  Messier  13,  reproduced 
in  Plates  XIX  and  XX.  The  principal  advantage  of  the 
reflector  in  such  work,  as  already  explained,  is  the  con- 
centration of  the  light-rays,  irrespective  of  their  color,  in 
a  single  focal  image. 

The  photographs  of  nebulae  obtained  by  Ritchey  with  the 
two-foot  reflector  show  in  a  remarkable  way  the  beauty  and 
delicacy  of  structure  which  characterize  these  objects.  It 
will  be  seen  from  the  illustrations  in  the  plates  that  the  nebulae 
are  of  many  types,  although  the  spiral  form  predominates. 
The  Great  Nebula  in  Orion  (Plate  XXI),  which  is  the  most 
brilliant  of  the  larger  nebulae,  is  of  irregular  form,  and 
marked  complexity  of  structure.  Of  very  different  pattern 
is  the  beautiful  nebula  in  Cygnus,  the  delicate  filamentous 
structure  of  which  is  admirably  shown  by  Ritchey's  photo- 
graph (Plate  LXXXVII).  In  the  nebulae  which  envelop  the 
stars  of  the  Pleiades  (Plate  LXXXVI)  two  very  different 
types  of  structure  are  shown ;  long  parallel  filaments  predomi- 
nate, but  there  may  also  be  seen  in  the  photograph  a  mass  of 
nebulosity  resembling  the  flame  of  a  torch  blown  by  the  wind. 
But  although,  as  we  shall  see,  evidences  may  be  found  of  the 
relationship  of  the  stars  in  these  nebulae  to  the  cloud-forms 
themselves,  the  spiral  nebulae  certainly  appeal  most  strongly 
to  the  imagination.  The  largest  of  these,  the  Great  Nebula  in 
Andromeda,  is  perhaps  the  most  interesting  object  in  the 
heavens  (Frontispiece).  Persistent  attempts  to  measure  the 
distance  of  this  nebula  from  the  Earth,  made  with  the  most 


DEVELOPMENT  OF  REFLECTING  TELESCOPE      45 

powerful  of  modern  instruments,  have  totally  failed.  We 
may  therefore  conclude  that  this  distance  is  almost  incon- 
ceivably great,  and  that  therefore  the  dimensions  of  the 
nebula  are  so  enormous  as  to  be  quite  beyond  comparison 
with  those  of  the  solar  system.  In  the  beautifully  defined 
spiral  character  of  this  object,  so  clearly  visible  on  the  photo- 
graph, although  beyond  recognition  in  visual  observations, 
we  seem  to  see  strong  indications  of  motion  with  respect  to 
the  center.  But  hitherto,  in  spite  of  the  careful  comparison 
of  photographs  made  many  years  apart,  no  evidence  of  such 
motion  has  been  detected.  This  fact  would  tend  to  confirm 
what  we  already  know  from  measurement,  namely,  that  the 
nebula  is  exceedingly  remote  from  the  Earth,  and  that  the 
phenomena  which  it  exhibits  are  on  a  gigantic  scale.  We 
cannot  doubt  that  the  component  parts  are  in  motion,  and 
that  in  the  course  of  time  evidences  of  this  motion  will  come 
to  light.  But  to  detect  them  it  is  certain  that  the  most 
powerful  instrumental  means  will  be  required,  and  that  long 
intervals  of  time  must  separate  the  photographs  which  are  to 
be  compared. 

The  Great  Nebula  in  Andromeda  thus  stands  as  the 
largest  representative  of  that  great  class  of  nebulae  which 
was  first  made  known  through  Lord  Rosse's  discovery  of  the 
spiral  nebula  in  the  Hunting  Dogs.  From  some  of  Ritchey's 
photographs  we  are  fortunate  in  being  able  to  illustrate  other 
spiral  nebulae,  which  differ  in  various  particulars,  but  in 
all  cases  show  clearly  the  spiral  structure  (Plates  LXXXIX 
and  XC).  As  already  stated,  Keeler's  photographic  investi- 
gations with  the  Crossley  reflector  have  shown  that  while  large 
objects  of  this  kind  are  comparatively  few,  the  sky  is  scattered 
over  with  an  immense  number  of  small  ones.  The  investi- 
gation of  these  nebulae,  with  the  great  reflecting  telescopes 
of  the  future,  should  lead  to  results  of  fundamental 
importance. 


CHAPTER  VII 
ELEMENTARY  PRINCIPLES  OF  SPECTRUM  ANALYSIS 

THE  problem  of  determining  the  nature  of  the  nebulae 
seemed  to  be  placed  beyond  solution  by  telescopic  means 
when  it  was  found  that  star  clusters  exist  in  which  the  stars  are 
so  densely  packed  that  they  cannot  be  separately  distinguished 
by  any  telescope.  A  photographic  illustration  of  this  is  given 
in  Plate  XIX.  In  Plate  XI  we  see  a  cluster  easily  resolved 
into  its  constituent  stars.  In  the  case  of  Messier  13,  however, 
the  photograph  here  reproduced  might  leave  some  doubt  on 
the  score  of  resolvability.1  Visual  observations,  better  com- 
petent than  photographic  ones  to  settle  this  particular  point, 
remove  the  doubt  in  the  present  instance.  But  other  clusters 
are  still  more  closely  crowded,  and  it  was  easy  to  believe  that 
the  unresolved  nebulae  might  be  objects  of  this  nature.  The 
structure  of  such  a  nebula  as  that  shown  in  Plate  XC  might 
also  be  supposed  to  favor  such  a  view.  Sir  William  Herschel, 
great  not  only  as  an  observer,  but  as  a  philosopher  who  looked 
deep  into  the  nature  of  things,  was  not  deceived  by  these 
circumstances,  and  persisted  in  his  belief  that  the  nebulae 
are  masses  of  uncondensed  gas,  differing  essentially  from 
clusters  of  stars.  As  evidence  of  the  uncertainty  which 
nevertheless  existed,  it  must  be  added  that  Sir  John  Herschel, 
though  himself  a  great  philosopher,  was  led  to  a  contrary 
conclusion.  For  him  no  nebula  existed  that  could  not  be 
resolved  with  a  sufficiently  powerful  telescope  into  a  congeries 
of  stars.  Under  these  circumstances  it  is  evident  that  some 
additional  means  of  analysis  must  be  called  upon  to  solve  the 
problem.  For  as  telescopes  increased  in  size  the  nebulae 
remained  unresolved,  showing  that  either  they  were  in  their 

1  Even  the  large-scale  photograph  in  Plate  XX  does  not  separate  the  closest  stars. 

46 


PRINCIPLES  or  SPECTRUM  ANALYSIS  47 

nature  unresolvable,  or  that  far  more  powerful  instruments 
would  be  required  to  reveal  their  constituent  parts. 

This  was  the  condition  of  affairs  when  Spencer  boldly 
took  issue  with  the  astronomers.  Convinced  that  the 
principle  of  evolution  must  operate  universally,  and  that 
the  stars  must  have  their  origin  in  the  still  unformed 
masses  of  the  nebulae,  he  ventured  to  question  the  con- 
clusion that  the  resolution  of  nebulae  into  stars  was  only 
a  matter  of  telescopic  power.  He  had  not  long  to  wait  for 
support,  for  at  this  juncture  a  new  method  of  research,  long 
previously  foreshadowed  by  Fraunhofer's  analysis  of  sunlight 
in  the  early  part  of  the  nineteenth  century,  suddenly  pro- 
claimed its  power  of  accomplishing  many  surprising  results. 

It  has  been  known  since  the  time  of  Newton  that  when 
sunlight  is  passed  through  a  prism,  it  is  spread  out  into  a 
band  containing  all  the  colors  of  the  rainbow.  In  Newton's 
experiments  the  sunlight  was  admitted  to  the  prism  through 
a  circular  hole,  and  he  consequently  failed  to  see  in  the 
colored  spectrum  any  of  those  breaks  or  dark  lines  that  were 
found  in  later  years  to  be  so  significant.  Fraunhofer,  on  the 
contrary,  examined  sunlight  which  reached  the  prism  from 
a  narrow  slit,  placed  at  a  considerable  distance.  He  was 
rewarded  by  the  discovery  of  a  large  number  of  dark  lines, 
differing  greatly  from  one  another  in  intensity,  and  irregularly 
distributed  through  the  spectrum.  He  measured  the  positions 
of  these  lines  in  the  spectrum  with  care,  and  designated  the 
more  striking  ones  with  the  letters  of  the  alphabet.  His 
designations  are  still  retained,  and  the  dark  lines  of  the  solar 
spectrum  are  still  called  the  Fraunhofer  lines.  But  of  the 
origin  of  these  lines  Fraunhofer  had  no  knowledge.  He 
found,  indeed,  that  the  lines  seen  in  sunlight,  while  present 
in  the  light  of  the  planets,  were  replaced  by  different  lines 
in  the  spectra  of  some  of  the  stars.  But  while  he  con- 
cluded that  the  cause  of  the  lines  did  not  reside  in  the 


48  STELLAR  EVOLUTION 

Earth's  atmosphere,  he  nevertheless  failed  to  discover  their 
true  explanation,  and  thus  did  not  perceive  the  possibili- 
ties of  the  science  of  spectrum  analysis. 

Let  us  consider  for  a  moment  what  happens  when  light 
is  passed  through  a  prism.  We  may  assume  the  light  to  be 
derived  from  the  glowing  filament  of  an  incandescent  lamp, 
placed  just  in  front  of  a  narrow  slit.  After  passing  through 
the  slit  a  (Fig.  1)  the  divergent  rays  fall  upon  the  lens  6; 


FIG.  i 

Passage  of  Rays  through  a  Prism 

which  renders  them  parallel,  and  is  known  as  the  collimating 
lens.  The  parallel  rays  now  meet  the  face  of  the  prism  c, 
through  which  they  are  transmitted.  After  passing  through 
the  prism  the  rays  fall  upon  the  lens  d,  precisely  similar  to 
the  collimating  lens,  which  forms  an  image  on  the  screen  e. 
Now,  when  light  strikes  a  prism  it  is  deviated  from  a 
straight  path,  and  the  amount  of  its  deviation  depends  upon 
the  color  of  the  light.  Yellow  light,  for  example,  is  deflected 
by  a  prism  more  than  red  light.  Green  light  is  deflected 
more  than  yellow  light,  blue  light  suffers  even  a  greater 
change  of  direction,  while  violet  light  is  deflected  most  of  all. 
It  is  thus  evident  that  if  the  light  from  the  incandescent 
lamp  were  pure  red,  and  contained  no  other  color,  we  should 
have  a  red  image  of  the  slit  at  R.  If  it  were  yellow,  a  yellow 
image  of  the  slit  would  be  formed  at  Y.  Green  light  would 
form  a  green  image  of  the  slit  at  Gr,  blue  light  a  blue  image 


PRINCIPLES  OF  SPECTRUM  ANALYSIS  49 

at  B,  and  violet  light  a  violet  image  at  V.  White  light  is 
compounded  of  all  these  colors,  and  shows  every  intermedi- 
ate gradation  of  tint.  When  passed  through  a  prism  it  is 
therefore  dispersed  into  a  colored  spectrum,  extending  from 
red  at  one  end  through  yellow,  green,  and  blue  to  violet.  This 
is  called  a  continuous  spectrum,  and  is  produced  when  the 
light  from  any  white-hot  solid  body  is  analyzed  by  a  prism. 
Liquids,  or  even  gases  when  sufficiently  compressed,  may 
give  a  continuous  spectrum  when  highly  heated.  But  vapors 
and  gases,  under  ordinary  conditions,  produce  characteristic 
spectra  of  bright  lines,  by  which  they  may  be  recognized. 

For  example,  let  us  replace  the  incandescent  lamp  flame 
by  a  non-luminous  gas  flame,  such  as  is  produced  when  gas 
is  burned  after  being  thoroughly  mixed  with  air.  If  we 
introduce  into  this  flame  a  little  common  salt,  it  will  be 
instantly  colored  a  deep  yellow.  This  yellow  light,  after 
transmission  through  the  slit  and  the  prism,  will  produce 
upon  the  screen  a  single  yellow  line  at  the  point  Y.  A  more 
powerful  instrument  would  resolve  this  line  into  two,  placed 
very  close  together  on  the  screen.  But  for  our  present  pur- 
poses we  may  consider  this  to  be  a  single  line  due  to  the 
metal  sodium,  which  in  conjunction  with  chlorine  constitutes 
common  salt.  Wherever  sodium  is  present  in  a  state  of  vapor, 
whether  in  a  flame,  or  between  the  carbon  poles  of  an  elec- 
tric arc,  or  in  the  atmosphere  of  the  Sun,  or  in  that  of  the 
most  distant  star,  it  gives  rise  to  this  line,  which  always  lies 
at  precisely  the  same  point  in  the  spectrum.  With  suffi- 
ciently powerful  instruments  the  line  is  always  double,  and 
its  presence,  when  accurately  determined,  is  sufficient  to 
prove  the  existence  of  sodium  in  any  luminous  source  (Fig.  1, 
Plate  XXII). 

Most  substances,  when  their  vapors  are  caused  to  radiate 
in  this  way,  produce  more  than  one  colored  image  of  the 
slit  upon  the  screen.  Thus  strontium,  when  introduced  into 


50  STELLAR  EVOLUTION 

the  flame,  gives  two  red  lines  and  a  strong  blue  line.  Potas- 
sium gives  a  line  in  the  extreme  red  and  another  in  the 
extreme  violet.  But  the  essential  point  to  notice  is  that  no 
two  substances  give  lines  at  precisely  the  same  place  in  the 
spectrum.  From  this  we  may  conclude  that  the  spectra  are 
entirely  characteristic  of  the  various  elements,  and  therefore 
that  the  presence  of  these  elements  in  a  state  of  vapor  can 
always  be  recognized  by  the  detection  of  their  peculiar  lines. 

The  spectra  of  the  elements  are  of  all  degrees  of  com- 
plexity, ranging  from  only  two  or  three  lines  up  to  several 
thousand.  Iron,  for  example,  when  turned  into  vapor  in 
the  electric  arc,  shows,  after  analysis  by  the  prism,  several 
thousand  lines,  irregularly  distributed  through  all  parts 
of  the  spectrum  (a  few  of  these  are  shown  in  Fig.  2,  Plate 
XXII) .  It  is  evident,  therefore,  if  the  lines  are  to  be  clearly 
distinguished  from  one  another,  and  so  accurately  recognized 
as  to  avoid  confusing  a  line  of  iron,  for  example,  with  one 
belonging  to  some  other  substance,  that  powerful  dispersion 
may  be  necessary;  i.  e.,  the  various  lines  must  be  separated 
from  one  another  as  far  as  possible  by  drawing  out  the  spec- 
trum to  a  great  length.  This  can  be  done  by  passing  the 
light  through  several  prisms  in  succession,  rather  than 
through  a  single  prism,  as  in  the  present  instance. 

So  far  we  have  referred  to  the  spectra  of  metallic  vapors, 
rendered  luminous  in  the  gas  flame  or  in  the  electric  arc. 
In  order  to  obtain  the  characteristic  spectrum  of  a  gas,  such 
as  hydrogen,  it  may  be  placed  in  a  tube,  and  made  lumi- 
nous by  an  electric  discharge.  The  best  results  are  ob- 
tained after  the  pressure  in  the  tube  has  been  reduced  by 
pumping  out  some  of  the  gas,  until  the  electric  discharge 
passes  quietly  and  continuously,  so  that  the  whole  interior 
of  the  tube  continues  to  glow  with  the  light  of  its  gaseous  con- 
tents. This  light,  when  analyzed  by  a  spectroscope  like  that 
shown  in  Fig.  2,  is  found  to  give  lines  which  are  character- 


PRINCIPLES  OF  SPECTRUM  ANALYSIS 


51 


istic  of  the  gas  employed.  The  light  of  hydrogen  in  a  vacuum 
tube,  for  example,  gives  precisely  the  same  spectrum  as  the 
light  of  hydrogen  proceeding  from  one  of  the  great  flames 
at  the  edge  of  the  Sun. 

We  have  now.  considered  two  types  of  spectra:  (1)  the 
continuous  spectrum,  produced  when  a  solid  body,  a  liquid,  or 


FIG.  2 

Kirchhoff's  Spectroscope 

a  highly  compressed  gas,  is  rendered  white-hot  by  sufficient 
heat;  and  (2)  a  bright-line  spectrum,  consisting  of  bright 
lines,  irregularly  distributed  on  a  dark  background,  and  de- 
rived from  the  prismatic  analysis  of  the  light  emitted  by 
luminous  metallic  vapors,  or  gases  rendered  incandescent 
by  electric  discharges.  One  other  type  of  spectrum  remains 
to  be  mentioned:  a  dark-line  spectrum,  such  as  Kirchhoff 
observed  and  explained  when  he  effected  his  famous  analysis 
of  sunlight  at  Heidelberg  in  1859. 

We  have  already  remarked  that  Fraunhofer  had  noted 


52  STELLAR  EVOLUTION 

the  existence  of  dark  lines  in  the  continuous  spectrum  of  the 
Sun,  and  accurately  measured  their  positions,  though  with- 
out understanding  their  meaning.  Kirchhoff,  using  the  four- 
prism  spectroscope  shown  in  Fig.  2,  saw  these  same  dark 
lines  in  the  solar  spectrum,  and  succeeded  in  explaining  their 
origin.  In  the  yellow  part  of  the  spectrum  he  observed  two 
strong  dark  lines,  very  close  together.  When  the  sunlight 
was  excluded  from  the  spectroscope,  and  a  gas  flame  contain- 
ing sodium  vapor  was  placed  in  front  of  the  slit,  two  strong 
bright  lines,  occupying  exactly  the  same  positions  as  the 
dark  lines  of  the  solar  spectrum,  were  seen  in  their  place. 
The  flame  was  then  copiously  charged  with  sodium  vapor 
and  retained  in  its  position  in  front  of  the  slit,  the  sunlight 
being  permitted  to  shine  through  it.  It  was  immediately 
noticed  that  the  two  dark  lines  in  the  solar  spectrum  were 
considerably  darker  and  more  conspicuous  when  the  sunlight 
passed  through  the  sodium  flame  than  when  it  was  observed 
alone.  Furthermore,  it  was  found  that  when  any  white  light, 
producing  a  continuous  spectrum  without  lines,  was  allowed 
to  shine  through  a  flame  containing  sodium  vapor,  the  effect 
of  the  flame  was  to  produce  two  dark  lines  in  the  yellow,  in 
the  precise  position  of  this  conspicuous  pair  of  dark  lines. 

Iron,  when  transformed  to  luminous  vapor  in  the  electric 
arc,  gave  an  even  more  convincing  proof  that  the  true  expla- 
nation of  the  solar  spectrum  had  been  found:  the  bright 
lines  observed  in  its  spectrum  by  Kirchhoff  and  Bunsen 
were  seen  to  be  represented  in  the  solar  spectrum  by  an 
equal  number  of  dark  lines,  precisely  resembling  them  both 
in  position  and  in  relative  intensity.  Magnesium,  nickel, 
calcium,  and  other  substances  gave  similar  results,  and  the 
conclusion  was  irresistible  that  all  of  these  substances  exist 
in  the  Sun  in  a  state  of  vapor.  It  followed  from  these  exper- 
iments that  the  body  of  the  Sun  must  be  an  intensely  hot 
mass,  emitting  white  light,  which,  if  it  could  be  observed 


PRINCIPLES  OF  SPECTBUM  ANALYSIS  53 

alone,  would  give  a  continuous  spectrum,  crossed  by  no  lines 
of  any  kind.  Surrounding  this  brilliant  white  sphere,  the 
observations  proved  the  existence  of  a  cooler  atmosphere  con- 
taining, in  a  state  of  vapor,  most  of  the  metals  known  on  the 
Earth.  These  vapors,  though  cooler  than  the  central  body 
of  the  Sun,  are  nevertheless  intensely  hot,  their  temperature 
undoubtedly  exceeding  that  of  the  most  powerful  electric  arc. 
Hence,  if  their  light  could  be  observed  alone,  they  would  be 
seen  to  give  a  very  complex  spectrum  of  bright  lines,  in  which 
all  of  the  lines  characteristic  of  the  different  elements  would 
be  present.  It  will  be  shown  later  that  such  a  spectrum  of 
bright  lines  may  be  seen  at  the  edge  of  the  Sun,  when  the 
apparatus  is  so  adjusted  as  to  admit  only  the  light  of  the 
chromosphere  to  the  slit  of  the  spectroscope,  while  excluding 
all  of  the  light  from  the  Sun's  disk.  The  bright  lines  in 
this  spectrum  are  less  brilliant  than  the  continuous  spectrum 
due  to  the  more  highly  heated  body  of  the  Sun.  Hence, 
when  observed  against  the  disk,  the  bright  lines,  appear 
dark  by  comparison.  The  cooler  metallic  vapors  were 
shown  by  KirchhofFs  experiments  to  be  capable  of  absorbing 
the  same  rays  which  they  themselves  emit,  and  the  feebler 
radiations,  emitted  by  the  vapors  themselves,  produce  the 
dark  lines  of  the  solar  spectrum.1  It  is  important  to  notice 
that  these  so-called  dark  lines  are  dark  only  by  comparison, 
since  it  will  be  explained  later  that  photographs  of  the  Sun 
can  be  taken  by  the  light  of  any  of  these  lines  with  the  spec- 
troheliograph,  showing  the  distribution  of  the  corresponding 
element  in  the  solar  atmosphere. 

It  immediately  became  evident  to  students  of  astrophysics 
that  the  method  of  analysis  initiated  by  Kirchhoff  must  prove 
immensely  powerful  in  extending  their  researches.  In  1862 
Huggins,  Secchi,  and  Rutherfurd  commenced  their  extensive 

!In  Fig.  1,  Plate  XXII,  the  two  bright  lines  are  due  to  very  hot  sodium  vapor  at 
the  center  of  the  arc.  The  cooler  and  less  dense  vapor  in  the  outer  arc  produces,  by 
absorption,  the  narrow  dark  lines  seen  superposed  on  the  bright  ones. 


54  STELLAK  EVOLUTION 

observations  on  the  spectra  of  stars,  and  soon  established  a 
system  of  types,  based  upon  the  examination  of  the  spectra 
of  several  thousand  objects.  This  work  has  since  been  greatly 
extended  through  the  application  of  photographic  methods, 
introduced  by  Huggins,  and  applied  with  marked  success  by 
Draper  and  many  others.  In  1868  the  spectroscope  was 
used  for  the  first  time,  to  analyze  the  red  flames  seen  during 
total  eclipses  of  the  Sun.  Not  only  did  it  demonstrate  their 
gaseous  nature,  but  a  short  time  later,  through  the  efforts  of 
Janssen,  Lockyer,  and  Huggins,  it  was  found  possible  to  em- 
ploy the  spectroscope  to  observe  the  forms  of  the  prominences 
in  full  sunlight. 

These  and  other  applications  of  the  spectroscope  will  be 
more  fully  described  in  subsequent  chapters.  Our  present 
purpose  is  to  explain  how  the  new  method,  in  the  hands  of 
Huggins  (Plate  XXIII),  finally  proved  beyond  doubt  that 
certain  nebulae  are  to  be  sharply  distinguished  from  star 
clusters. 

Sir  William  Huggins'  account  of  his  first  spectroscopic 
examination  of  a  nebula  is  recorded  in  the  Publications  of  the 
Tulse  Hill  Observatory,  Vol.  I: 

On  the  evening  of  August  29,  1864,  I  directed  the  spectroscope 
for  the  first  time  to  a  planetary  nebula  in  Draco.  I  looked  into  the 
spectroscope.  No  spectrum  such  as  I  had  expected!  A  single 
bright  line  only!  At  first  I  suspected  some  displacement  of  the 
prism  and  that  I  was  looking  at  a  reflection  of  the  illuminated  slit 
from  one  of  its  faces.  This  thought  was  scarcely  more  than 
momentary;  then  the  true  interpretation  flashed  upon  me.  The 
light  of  the  nebula  was  monochromatic,  and  so,  unlike  any  other 
light  I  had  as  yet  subjected  to  prismatic  examination,  could  not  be 
extended  out  to  form  a  complete  spectrum.  After  passing  through 
the  two  prisms  it  remained  concentrated  into  a  single  bright  line, 
having  a  width  corresponding  to  the  width  of  the  slit,  and  occupy- 
ing in  the  instrument  a  position  at  that  part  of  the  spectrum  to 
which  its  light  belongs  in  refrangibility.  A  little  closer  looking 
showed  two  other  bright  lines  on  the  side  toward  the  blue,  all  three 


PRINCIPLES  OF  SPECTRUM  ANALYSIS  55 

lines  being  separated  by  intervals  relatively  dark.  The  riddle  of 
the  nebulae  was  solved.  The  answer,  which  had  come  to  us  in  the 
light  itself,  read:  Not  an  aggregation  of  stars,  but  a  luminous  gas. 

With  this  advance  a  new  era  of  progress  began.  The 
power  of  the  spectroscope  to  distinguish  between  a  glowing 
gas  and  a  starlike  mass  of  partially  condensed  vapors  estab- 
lished it  at  once  in  the  place  it  still  holds  as  the  chief  instru- 
ment of  the  student  of  stellar  evolution.  It  became  apparent 
that  the  unformed  nebulae  might  furnish  the  material  from 
which  stars  are  made. 

It  must  not  be  forgotten,  however,  that  only  a  small 
number  of  nebulae  give  a  spectrum  of  bright  lines,  showing 
them  to  be  gaseous.  Most  of  the  nebulae,  including  the 
very  numerous  spiral  type,  have  a  continuous  spectrum,  in 
which  no  lines  have  yet  been  detected.  As  stars  are  almost 
certainly  formed  from  these  "white"  nebulae,  as  well  as 
from  the  "green"  gaseous  ones,  the  theory  of  stellar  evolu- 
tion must  be  broad  enough  to  embrace  both  types. 


CHAPTER  VIII 

GRATING  SPECTROSCOPES  AND  THE  CHEMICAL 
COMPOSITION  OP  THE  SUN 

THE  general  process  employed  by  Kirchhoff  to  investi- 
gate the  chemical  constitution  of  the  Sun  has  already 
been  described,  but  it  also  seems  desirable  to  give  an 
account  of  the  perfected  method  used  for  this  purpose  in 
a  modern  laboratory.  In  order  to  prove  that  a  given  sub-- 
stance  exists  in  the  Sun,  its  lines  must  be  identified  with 
certainty  in  the  solar  spectrum.  The  spectrum  of  iron,  for 
example,  contains  thousands  of  lines,  and  it  might  easily 
happen  that  through  chance  proximity  many  of  these  lines 
would  appear  to  coincide  with  some  of  the  exceedingly 
numerous  lines  of  the  solar  spectrum.  It  is  evident,  there- 
fore, that  the  method  of  comparison  adopted  must  be  such 
as  to  permit  of  a  high  degree  of  precision  in  measuring  the 
positions  of  the  lines.  In  other  words,  the  dispersion  of  the 
spectroscope  must  be  so  great  as  to  give  a  very  long  spec- 
trum, in  which  the  lines  are  well  separated  from  one  another. 
Thus  their  positions  can  be  accurately  determined,  and  there  is 
no  danger  of  confusion  in  the  case  of  closely  adjacent  lines, 
which  in  a  less  powerful  instrument  might  be  seen  as  one. 

The  recent  great  advances  in  spectroscopy  have  been  due 
in  very  large  measure  to  the  success  of  Rowland  in  ruling 
gratings  of  high  resolving  power.  In  a  previous  chapter  it 
was  remarked  that  the  dispersion  of  a  spectroscope  may  be 
increased  by  increasing  the  number  of  prisms  through  which 
the  light  passes.  This  not  only  gives  a  longer  spectrum;  it 
also  increases  the  resolving  power  of  the  instrument,  or 
its  capacity  of  separating  closely  adjacent  lines.  But 

56 


GRATING  SPECTROSCOPES  57 

through  the  loss  of  light  by  reflection  and  absorption,  which 
becomes  very  serious  when  many  prisms  are  employed, 
a  limit  is  soon  set  to  the  increase  in  resolving  power  of 
prism  spectroscopes.  It  is  for  this  reason  that  the  grating 
has  played  so  large  a  part  in  the  recent  development  of 
the  subject.  For  the  resolving  power  of  a  perfect  grating 
depends  only  upon  the  total  number  of  lines  it  contains,  and 
the  light  efficiency,  per  unit  area,  may  be  as  great  for  a 
large  grating  as  for  a  small  one. 

The  production  of  very  powerful  spectroscopes,  through 
the  use  of  large  and  accurately  ruled  gratings,  is  what  Row- 
land succeeded  in  accomplishing  in  his  epoch-making  work 
at  the  Johns  Hopkins  University.  An  optical  grating  con- 
sists of  a  polished  metallic  surface,  on  which  many  equidis- 
tant lines  are  ruled  with  a  diamond  point.  The  perfection 
of  the  spectra  given  by  such  a  grating  depends  upon  the 
number  of  lines  it  contains  and  upon  the  accuracy  of  their 
spacing.  The  difficulty  of  Rowland's  task  will  be  appre- 
ciated when  it  is  remembered  that  a  grating  must  contain 
from  10,000  to  20,000  lines  per  inch,  and  that  errors  in  the 
positions  of  the  lines,  amounting  to  a  very  small  fraction  of 
the  interval  between  them,  would  affect  the  performance  of 
the  grating,  tending  to  blur  and  confuse  the  spectra  pro- 
duced by  it. 

Gratings  that  gave  very  good  results  were  made  many 
years  ago  by  Rutherfurd,  of  New  York,  but  it  remained  for 
Rowland  to  surpass  them,  both  in  quality  and  in  size.  His 
celebrated  ruling-engines  (Plate  XXIV),  which  are  still  in 
regular  use  in  the  underground  constant-temperature  vaults 
of  the  physical  laboratory  at  the  Johns  Hopkins  University, 
depend  for  their  success  upon  the  fact  that  the  screw,  which 
is  employed  to  move  the  grating-plate  forward  by  about 
1/15,000  of  an  inch  between  successive  strokes  of  the  dia- 
mond, contains  almost  no  errors.  It  cannot  be  said,  of  course, 


58  STELLAR  EVOLUTION 

that  the  screw  is  entirely  free  from  error,  but  the  effect  of 
the  exceedingly  minute  irregularities  is  almost  wholly  com- 
pensated by  ingenious  devices  that  form  a  part  of  the  rul- 
ing-engine. The  machine  is  automatic  in  its  action,  and 
when  set  in  motion  the  ruling  of  a  large  grating  goes  on 
without  interruption  for  six  days  and  nights  before  it  is 
completed. 

The  gratings  manufactured  on  Rowland's  machine  have 
gone  into  observatories  and  laboratories  in  all  parts  of  the 
world,  where  they  have  been  the  principal  agents  of  spectro- 
scopic  research  during  the  last  quarter  of  a  century.  Their 
great  efficiency  has  caused  them  to  displace  prisms  from 
nearly  all  spectroscopes  in  which  very  high  resolving  power 
is  required.  As  we  shall  see  later,  however,  the  prism  still 
remains  of  great  importance  to  the  spectroscopist,  particularly 
in  work  requiring  moderate  resolving  power,  where  it  gives 
a  much  brighter  spectrum  than  a  grating. 

Rowland's  contributions  to  spectroscopy  were  by  no  means 
confined  to  the  manufacture  and  distribution  of  optical  grat- 
ings. In  addition  to  his  very  extensive  researches  on  the 
solar  spectrum,  and  on  the  spectra  of  the  elements,  he 
invented  the  concave  grating,  which  now  forms  an  essential 
part  of  the  powerful  spectroscopes  found  in  many  labora- 
tories. Prior  to  Rowland's  time  the  comparatively  few 
gratings  which  had  been  made  were  ruled  on  plane  surfaces, 
and  employed  with  the  ordinary  collimator  and  telescope 
of  the  laboratory  spectroscope.  That  is  to  say,  the  prism  of 
an  ordinary  spectroscope  was  removed,  and  the  grating  sub- 
stituted for  it.  In  such  an  instrument  the  rays  of  light,  after 
passing  through  the  slit,  fall  upon  the  collimator  lens,  which 
renders  them  parallel.  The  parallel  rays  then  meet  the  sur- 
face of  the  grating,  where  they  are  diffracted  and  spread  out 
into  a  spectrum.  This  spectrum  is  observed  or  photographed 
with  the  aid  of  a  second  lens,  which  forms  its  image  on  the 


GEATING  SPECTROSCOPES 


59 


retina  or  on  a  sensitive  plate.  A  large  spectroscope  of  this 
kind,  used  with  the  40-inch  Yerkes  telescope  for  spectroscopic 
observations  of  the  Sun,  is  illustrated  in  Plate  XXX. 

Rowland  showed,  from  theoretical  considerations,  that, 
if  the  grating  were  given  a  concave  spherical  surface,  the 
collimator  lens,  and  the  observ- 
ing telescope  as  well,  might  be 
entirely  dispensed  with.  He 
also  devised  the  form  of  mount- 
ing for  a  concave  grating  illus- 
trated in  Fig.  3.  In  the  diagram, 
a  is  the  slit  through  which  the 
light  enters,  b  the  concave  grat- 
ing, and  c  the  eye  or  photo- 
graphic plate.  It  will  be  seen 
that  no  lenses  enter  into  the 
construction  of  the  apparatus; 
for  some  classes  of  work  this  is 
a  point  of  great  advantage.  In 

the  largest  gratings  used  by  Rowland  the  radius  of  curvature 
of  the  grating-plate,  which  is  equal  to  the  distance  between 
the  grating  6  and  the  photographic  plate  c,  is  21  feet.  The 
spectrum  given  by  such  a  grating  is  many  feet  in  length, 
and  a  portion  of  the  spectrum  20  inches  long  or  longer  can 
be  recorded  by  a  single  exposure  on  the  photographic  plate. 
In  order  to  pass  from  one  part  of  the  spectrum  to  another, 
the  grating-carriage  6  is  moved  along  the  rail  a&,  which 
causes  the  plate-carriage  c  to  move  toward  or  away  from  the 
slit  on  the  rail  ac.  The  whole  apparatus  is  set  up  on  piers 
in  a  dark  room,  to  which  no  light  is  admitted  except  that 
which  passes  through  the  slit  of  the  spectroscope. 

It  should  be  remarked  that  a  grating,  unlike  a  prism, 
produces  not  merely  a  single  spectrum,  but  several  spectra, 
which  can  be  observed  successively  by  moving  the  carriage  c 


FIG.  3 

Diagram  of  a  Concave  Grating 
Mounting 


60  STELLAR  EVOLUTION 

along  the  track  away  from  the  slit.  The  first-order  spec- 
trum lies  nearest  the  slit.  The  second-order  spectrum,  twice 
as  long  as  the  first,  which  it  partially  overlaps,  lies  farther 
from  the  slit.  The  third  and  fourth  orders,  of  increasingly 
higher  dispersion,  lie  still  farther  from  the  slit.  Only  a  por- 
tion of  the  fifth  order  can  be  observed  with  this  instrument, 
and  the  higher  orders,  also  beyond  reach,  are  usually  too 
faint  to  be  of  any  service. 

Let  us  suppose  that  we  wish  to  determine  with  such  a 
spectroscope  whether  iron  exists  in  the  Sun.  To  accomplish 
this,  sunlight  must  be  reflected  from  the  mirror  of  a  heliostat 
(driven  by  clock-work,  to  maintain  the  beam  in  a  fixed  direc- 
tion) to  the  slit.  Between  the  slit  and  the  heliostat  a  lens  is 
introduced,  for  the  purpose  of  forming  an  image  of  the  Sun 
upon  the  slit.  When  the  illumination  is  secured  in  this  way, 
the  whole  grating  is  filled  with  light  from  the  diverging  rays. 
The  grating  then  produces  an  image  of  the  solar  spectrum 
upon  the  photographic  plate,  where  it  may  be  recorded  by 
giving  a  suitable  exposure. 

To  facilitate  an  accurate  comparison,  the  solar  spectrum 
is  photographed  side  by  side  on  the  same  plate  with  the 
spectrum  of  the  substance  whose  presence  in  the  Sun  is  to 
be  determined.  In  order  to  accomplish  this,  one-half  of  the 
slit  is  covered,  and  the  sunlight  is  admitted  through  the 
other  half.  Thus  the  solar  spectrum  is  photographed  on 
one  side  of  the  plate.  After  this  exposure  is  completed,  the 
sunlight  is  shut  off,  and  the  screen  in  front  of  the  slit  moved 
so  as  to  cover  the  half  previously  open,  and  to  uncover  the 
other  half.  The  image  of  the  Sun  on  the  slit  of  the  spectro- 
scope is  then  replaced  by  an  image  of  an  electric  arc  light, 
burning  between  two  poles  of  iron.  The  spectrum  of  the  iron 
vapor  is  thus  produced  on  the  plate,  and  a  strip  of  this 
spectrum  is  photographed  beside  the  strip  of  solar  spectrum. 
This  is  illustrated  in  Fig.  2,  Plate  XXII,  where  the  upper 


GRATING  SPECTROSCOPES  61 

strip  is  a  small  part  of  the  spectrum  of  iron.  It  will  be  seen 
by  a  glance  at  this  photograph  that  these  bright  lines  of  iron 
are  represented  in  the  solar  spectrum  by  corresponding  dark 
lines,  which  accurately  match  them  in  position.  In  Rowland's 
work  on  the  solar  spectrum  thousands  of  lines  were  found  to 
correspond  with  iron  lines  given  by  the  electric  arc. 

The  same  process  can  be  employed  to  determine  the  pres- 
ence of  other  substances  in  the  Sun.  In  the  case  of  metals, 
the  electric  discharge  may  be  caused  to  pass  between  two 
metallic  rods,  or  fragments  of  the  metal  may  be  placed  in  a 
hole  drilled  in  one  of  the  carbons  of  an  ordinary  electric  arc- 
lamp.  In  the  latter  case  the  spectrum  of  carbon,  and  also  of 
the  impurities  which  the  carbon  poles  always  contain,  will 
appear  on  the  plate  with  the  spectrum  of  the  metal  in  ques- 
tion. But  these  extra  lines  may  always  be  identified,  and 
usually  give  no  trouble.  The  identification  of  the  solar 
lines,  however,  is  not  always  so  simple  as  in  the  case  of  iron. 
Many  substances  are  doubtfully  represented  in  the  Sun  by 
only  a  small  number  of  lines,  and  it  is  sometimes  very 
difficult  to  decide  whether  a  few  apparent  coincidences  are 
sufficient  to  warrant  one  in  drawing  definite  conclusions. 
The  matter  is  usually  determined  by  ascertaining  whether 
certain  well-known  groups  of  lines,  which  for  various  reasons 
are  considered  to  be  especially  characteristic  of  an  element, 
are  actually  represented.  If  these  groups  are  absent,  an 
apparent  coincidence  with  certain  less  characteristic  lines 
belonging  to  the  same  element  should  be  regarded  with 
suspicion.  In  the  case  of  gases,  the  comparison  is  effected 
by  the  aid  of  vacuum  tubes,  in  which  the  gas,  usually  at  low 
pressure,  is  illuminated  by  an  electric  discharge.  Thus  the 
lines  given  by  a  hydrogen  tube  in  the  laboratory  have  been 
shown  to  coincide  in  position  with  lines  ascribed  to  hydrogen 
in  the  Sun. 

After  many  years  of  study  of  the  solar  spectrum  by  these 


62  STELLAR  EVOLUTION 

methods,  Eowland  reached  the  conclusion  that  the  chemical 
composition  of  the  Sun  closely  resembles  that  of  the  Earth. 
Certain  elements,  such  as  gold  and  radium,  iodine,  sulphur, 
and  phosphorus,  chlorine  and  nitrogen,  have  not  been 
detected  in  the  Sun.  But  this  does  not  prove  that  they  are 
certainly  absent,  as  their  level  in  the  solar  atmosphere,  or 
the  low  degree  of  their  absorptive  effects  might  prevent 
them  from  being  represented.  On  the  other  hand,  various 
substances,  not  yet  found  on  the  Earth,  are  shown  by  many 
unidentified  lines  of  the  solar  spectrum  to  be  present  in  the 
Sun.  Some,  if  not  all,  of  these,  will  probably  be  discovered 
by  chemists,  just  as  helium  was  found  by  Ramsay  in  cleveite 
(p.  78).  Eowland  remarked  that  if  the  Earth  were  heated 
to  a  sufficiently  high  temperature,  it  would  give  a  spectrum 
closely  resembling  that  of  the  Sun. 

The  most  perfect  maps  of  the  solar  spectrum  are  those  of 
Rowland  and  Higgs.  These  are  enlarged  from  photographs 
made  with  the  concave  grating,  and  contain  an  immense 
number  of  lines.  Both  maps  extend  into  the  extreme  ultra- 
violet spectrum  (the  invisible  region  beyond  the  violet),  and 
that  of  Higgs  includes  a  considerable  region  of  the  infra-red 
(also  invisible  to  the  eye)  where  photographic  plates  sensi- 
tized for  red  light  with  alizarin  blue  or  other  dyes  must  be 
employed.  Both  maps  are  provided  with  scales  of  wave- 
length, so  that  the  approximate  positions  of  the  lines  can 
be  read  off  at  once.  The  precise  positions  of  all  solar  lines 
photographed  by  Rowland  are  given  in  his  Preliminary 
Table  of  Solar  Spectrum  Wave-Lengths,  which  records  the 
places  of  about  20,000  lines.  This  table,  although  known 
to  contain  some  small  errors,  is  at  present  employed  by  all 
spectroscopists  as  the  standard  of  reference.  It  gives  Row- 
land's identifications  of  the  solar  lines,  but  about  two- 
thirds  of  the  lines  have  not  yet  been  referred  to  any  known 
element.  Recent  investigations  of  the  spectra  of  various 


GRATING  SPECTROSCOPES  63 

metals  will  no  doubt  permit  a  considerable  number  of  these 
lines  to  be  identified. 

In  any  examination  of  the  solar  spectrum  the  observer 
cannot  fail  to  be  struck  by  the  changing  appearance  of  the 
lines  in  certain  regions.  In  the  yellow  part  of  the  spectrum, 
for  example,  near  the  well-known  D  lines  of  sodium,  the 
most  casual  examination  will  show  surprising  variations  in 
the  intensity  of  the  countless  lines  which  are  frequently 
conspicuous  here.  How  great  the  change  is  may  be  seen 
in  Plate  XXV,  which  is  a  reproduction  of  two  photographs 
of  this  part  of  the  spectrum  taken  under  different  conditions. 
The  lines  which  thus  change  in  intensity  are  called  telluric 
lines,  since  they  are  due  to  the  absorption  of  the  gases  in 
the  Earth's  atmosphere.  The  region  illustrated  in  Plate 
XXV  contains  a  large  number  of  lines  due  to  water  vapor. 
Since  the  amount  of  water  vapor  undergoes  great  variations, 
it  is  natural  that  the  intensities  of  the  lines  should  change 
accordingly. 

All  of  the  telluric  lines  are  most  conspicuous  in  the 
spectrum  of  the  Sun  when  it  is  near  the  horizon,  since  in  this 
case  the  light  traverses  a  very  great  depth  of  atmosphere 
before  it  reaches  the  spectroscope.  Photographs  of  the 
spectrum  of  the  high  and  low  Sun  might  therefore  be 
expected  to  show  marked  differences  in  the  intensity  of  the 
telluric  lines.  This  is  actually  the  case,  and  the  method 
therefore  affords  one  means  of  identifying  lines  due  to  the 
absorption  of  our  atmosphere.  The  oxygen  in  the  air  pro- 
duces two  similar  groups  (A  and  B  in  Fraunhofer's  original 
designation  of  the  solar  lines)  which  lie  at  the  extreme  red 
end  of  the  solar  spectrum.  Cornu  observed  these  same 
groups  in  the  spectrum  of  an  electric  light  at  the  summit  of 
the  Eiffel  Tower,  as  seen  from  the  [Ecole  Polytechnique  in 
Paris,  at  a  distance  of  about  2.7  miles. 

An  ingenious  method  was  employed  by  Cornu  to  distin- 


64  STELLAR  EVOLUTION 

guisli  the  telluric  lines  from  those  due  to  absorption  in  the 
Sun's  atmosphere.  According  to  Doppler's  principle,  the 
lines  in  the  spectrum  of  the  east  limb  of  the  Sun  must  be 
displaced  toward  the  violet  ^by  motion  of  approach),  and 
those  from  the  west  limb  toward  the  red  (recession),  since  the 
Sun  is  rotating  on  its  axis  in  a  period  of  about  twenty-five 
days.  It  occurred  to  Cornu  that  this  fact  might  give  a  very 
delicate  means  of  picking  out  the  telluric  lines,  since  only  the 
lines  of  solar  origin  can  be  displaced  by  the  Sun's  rotation, 
while  those  due  to  absorption  in  the  Earth's  atmosphere 
remain  in  their  normal  positions.  He  formed  a  small  image 
of  the  Sun  on  the  slit  of  his  spectroscope,  by  means  of  a  lens 
which  could  be  made  to  oscillate  rapidly,  thus  causing  the  east 
limb  and  the  west  limb  of  the  Sun's  image  to  fall  alternately 
upon  the  slit.  If  the  spectrum  is  observed  while  the  image 
is  oscillating,  the  lines  of  solar  origin  will  be  seen  to  move 
rapidly  to  and  fro  through  a  short  distance,  while  the  telluric 
lines  will  remain  fixed.  This  method  was  successfully  em- 
ployed by  Cornu  in  an  important  study  of  the  telluric  lines. 
Other  investigations  of  these  lines,  which  have  resulted  in 
the  production  of  extensive  maps,  have  been  made  by  Thollon 
(continued,  by  Spee),  Becker,  and  McClean.  In  these  inves- 
tigations the  telluric  lines  were  distinguished  by  observations 
of  the  spectrum  of  the  high  and  low  Sun. 

If  passage  of  sunlight  through  our  atmosphere  is  thus 
competent  to  produce  dark  lines  in  the  solar  spectrum,  it  is 
evident  that  the  sunlight  reflected  from  a  planet  should  show 
evidence  of  its  double  transmission  through  the  planet's 
atmosphere.  This  method  is  actually  employed  to  determine 
the  presence  and  the  composition  of  the  atmospheres  of  the 
planets. 

Remarkable  as  was  Rowland's  success  in  the  manufacture 
of  gratings,  and  the  measurement  of  wave-lengths  with  their 
aid,  it  has  recently  been  surpassed  by  Michelson.  With  the 


GRATING  SPECTROSCOPES  65 

interferometer,  an  instrument  of  his  invention,  Michelson 
has  established  the  length  of  the  standard  meter  of  the  Inter- 
national Bureau  of  Weights  and  Measures,  in  terms  of  light- 
waves. This  fixes,  with  the  greatest  precision,  the  wave- 
length of  certain  lines  in  the  spectrum  of  cadmium,  and  these 
wave-lengths  were  adopted  at  the  Oxford  meeting  ( 1905)  of 
the  International  Union  for  Co-operation  in  Solar  Research 
as  primary  standards,  on  which  a  new  system  of  wave- 
lengths, to  replace  Rowland's  system,  will  be  based.  Through 
his  invention  of  the  echelon,  Michelson  has  realized  a  new 
form  of  grating,  composed  of  a  series  of  glass  plates,  precisely 
equal  in  thickness,  piled  one  on  another  like  a  flight  of  steps, 
through  which  a  parallel  beam  of  light  is  transmitted.  The 
spectra  thus  produced  are  of  a  very  high  order,  and  the 
resolving  power  surpasses  that  of  Rowland's  largest  gratings. 
The  echelon  spectroscope  thus  furnishes  the  means  of  analyz- 
ing compound  lines,  the  members  of  which  lie  so  close  to- 
gether that  they  cannot  be  separated  with  other  instruments. 
The  latest  success  achieved  by  Michelson,  however,  opens 
up  still  greater  possibilities  in  spectroscopy.  The  echelon 
can  be  used  only  for  the  study  of  narrow  and  sharply  defined 
lines;  its  application  is  therefore  limited  to  certain  special 
problems.  For  more  general  work,  both  in  the  laboratory 
and  in  the  solar  observatory,  very  large  gratings,  of  high 
resolving  power,  are  required.  Six-inch  gratings  (ruled  on 
a  disk  of  speculum  metal  6  inches  in  diameter)  were  success- 
fully made  by  Rowland.  After  several  years  of  labor 
Michelson  has  completed  a  ruling-machine  with  an  almost 
perfect  screw,  on  which  he  has  already  made  8-inch  and  10- 
inch  gratings.  He  hopes  to  produce  a  14-inch  grating,  the 
largest  for  which  his  machine  is  designed.  There  is  reason 
to  believe  that  his  plan  for  constructing  a  ruling-machine 
with  four  screws,  which  should  reduce  the  error  to  one-fourth 
its  amount  in  a  single  screw  machine,  would  result  in  the 


66  STELLAE  EVOLUTION 

production  of  good  20-inch  gratings.  The  enormous  impor- 
tance of  such  gratings,  in  their  application  to  the  study  of  the 
Sun,  will  become  clearer  as  we  proceed.  One  difficulty  to 
be  overcome  will  be  recognized  when  it  is  remembered  that 
a  20-inch  grating,  having  12,500  lines  to  the  inch,  would 
contain  more  than  2,000,000  lines,  each  about  10  inches  long. 
The  microscopic  diamond  crystal,  used  to  cut  all  these  lines 
in  the  hard  surface  of  the  speculum  metal,  must  not  break,  or 
change  its  form  appreciably,  during  the  entire  period  of  the 
work. 

It  is  satisfactory  to  add  that  Jewell  has  recently  con- 
structed a  new  ruling  machine  at  the  Johns  Hopkins  Univer- 
sity which  appears  likely,  from  preliminary  tests,  to  be  supe- 
rior to  Rowland's.  We  thus  have  good  reason  to  hope  that 
the  best  existing  photographs  of  the  solar  spectrum  will  soon 
be  surpassed. 


CHAPTER  IX 

PHENOMENA  OF  THE  SUN'S  SURFACE 

THE  results  described  in  the  last  chapter  relate  to  the 
light  of  the  Sun  as  a  whole,  and  not  to  the  details  of  its  sur- 
face phenomena.  In  most  of  the  investigations  there  de- 
scribed similar  results  might  be  attained  if  the  Sun  were 
removed  to  the  distance  of  the  nearer  stars.  In  that  case  it 
would  no  longer  be  possible,  even  with  the  most  powerful 
telescope,  to  detect  an  appreciable  disk,  and  the  solar  image 
would  be  reduced  to  a  microscopic  point,  brilliant  enough, 
however,  to  afford  sufficient  light  for  spectroscopic  examina- 
tion. But  it  has  already  been  pointed  out  that  investigations 
of  the  Sun  acquire  their  greatest  importance  through  the 
comparative  proximity  of  this  star  to  the  Earth.  All  other 
stars  are  so  far  away  that  no  distinction  can  be  drawn  be- 
tween the  radiations  characteristic  of  different  parts  of  their 
disks.  The  spectroscopist  must  therefore  be  content  to 
observe  in  such  cases  a  composite  spectrum,  produced  by  the 
superposition  of  the  spectra  of  the  various  surface  phenom- 
ena. The  Sun,  on  the  other  hand,  is  so  near  us  that  its 
image  at  the  focus  of  a  powerful  telescope  may  have  a  diam- 
eter as  great  as  7  inches,  or  even  greater.1  Consequently, 
the  light  from  any  point  in  this  image,  corresponding  to  a 
small  area  of  the  solar  surface,  can  be  studied  by  itself.  Our 
extensive  knowledge  of  the  Sun,  except  that  which  has  been 
derived  from  an  examination  of  its  light  as  a  whole,  is  based 
upon  this  fact. 

The  appearance  of  the  Sun  in  a  telescope  is  illustrated  by 
Plate  II,  which  is  a  reproduction  of  a  direct  photograph. 

i  The  actual  diameter  of  the  Sun  is  about  860,000  miles. 

67 


STELLAR  EVOLUTION 


The  Sun's  light  is  too  brilliant  to  permit  of  visual  observa- 
tion without  some  method  of  reducing  its  intensity.  The  best 
means  of  accomplishing  this  is  by  the  aid  of  the  polarizing 
helioscope,  which  is  attached  just  in  front  of  the  eye- 
piece of  the  telescope.  The  cone  of  light  from  the  object- 
glass  meets  a  plane  surface  in  the  helioscope,  from  which  it 
is  reflected  at  an  angle  such  as  to  polarize  the  rays.  As  is 
well  known,  the  amount  of  plane  polarized  light  which  can 
be  reflected  from  a  second  surface  depends  upon  the  angle 
at  which  the  rays  meet  this  surface.  Consequently,  by 
rotating  the  reflecting  prism  the  amount  of  light  which 
reaches  the  eye  can  be  varied  at  will,  thus  producing  an 
image  of  any  desired  brightness.  When  protected  by  this 
device,  the  eye  of  the  observer  of  solar  phenomena  is  sub- 
jected to  even  less  strain  than  is  frequently  experienced  in 
work  on  fainter  objects. 

A  casual  glance  at  the  solar  image  is  sufficient  to  show 
that  it  is  much  darker  near  the  circumference  than  at  the  cen- 
tral part  of  the  disk.  This  falling-off  in  brightness  toward 
the  limb  is  probably  due  to  the  absorption  of  a  smoke-like 
envelope,  which  completely  incloses  the  Sun.  The  absorp- 
tion is  so  marked  that  near  the  circumference  of  the  Sun  only 
about  13  per  cent,  of  the  violet  rays  escape.  For  the  blue, 
green,  and  yellow  rays  the  percentage  of  transmitted  light 
increases  progressively,  until  it  amounts  to  about  30  per  cent, 
for  the  red.  It  has  therefore  been  concluded  that,  if  this 
absorbing  atmosphere  were  removed,  the  color  of  the  Sun 
would  appear  blue,  since  the  intensity  of  the  violet  rays 
would  be  about  two  and  one -half  times  as  great  as  at  present, 
while  the  red  rays  would  be  only  about  half  again  as  bright 
as  they  are  now. 

The  visible  phenomena  of  the  Sun's  disk  include  the  sun- 
spots  and  the  faculae.  The  general  appearance  of  sun-spots, 
when  seen  with  a  low  magnifying  power,  is  shown  in  Plate  II. 


PHENOMENA  OF  THE  SUN'S  SUKFACE  69 

Under  perfect  atmospheric  conditions,  a  large  sun-spot,  when 
observed  with  a  powerful  telescope,  would  more  closely 
resemble  Plate  XXVI,  which  is  reproduced  from  a  drawing 
made  by  Langley.  The  best  solar  observers  agree  that  this 
drawing  is  one  of  the  most  perfect  representations  of  spot 
structure  yet  obtained.  The  long  narrow  filaments,  which 
constitute  the  penumbra  of  the  spot,  reach  in  toward  a  dark 
central  region,  called  the  umbra.  It  must  be  remembered 
that  the  darkness  of  the  umbra  is  only  relative:  if  observed 
alone,  and  not  in  contrast  with  the  more  brilliant  surround- 
ings of  the  photosphere,  the  great  brilliancy  of  the  umbra, 
surpassing  that  of  the  most  powerful  electric  arc-light,  would 
be  evident.  Knowledge  of  this  fact  has  been  quite  suf- 
ficient to  set  at  rest  the  old  notion  that  sun-spots  are 
merely  rents  in  a  brilliant  cloud-covering  of  the  Sun,  through 
which  a  dark  and  cool  interior  may  be  seen. 

According  to  Langley 's  view,  the  filaments  which,  taken 
together,  constitute  the  penumbra  are  everywhere  present 
on  the  solar  surface.  He  regarded  them  as  resembling  the 
stalks  of  a  wheat-field,  seen  on  end  in  the  undisturbed  pho- 
tosphere, and  revealing  more  of  their  true  characteristics  in 
the  penumbra,  where  they  are  bent  over  and  drawn  out 
toward  the  central  part  of  the  spot.  Langley  believed  that 
we  are  observing  columns  of  luminous  vapors  rising  from  the 
Sun's  interior,  the  seats  of  convection  currents  which  bring 
to  the  surface  the  immense  supplies  of  heat  radiated  by  the 
Sun  into  space.  Separating  these  luminous  columns  are 
darker  regions,  characterized  by  a  lower  degree  of  radiation. 

Such  minute  details  can  be  recorded  only  with  the  greatest 
difficulty.  Under  ordinary  atmospheric  conditions  the  solar 
image  is  not  seen  as  a  sharp  and  well-defined  object,  but  its 
details  are  continually  blurred  by  the  effect  of  irregularly 
heated  currents  in  our  atmosphere.  Even  under  the  best 
conditions  the  moments  of  very  sharp  definition  are  few,  and 


70  STELLAR  EVOLUTION 

the  greatest  patience  and  perseverance  are  required  on  the 
part  of  an  observer  who  would  record  his  impressions  of  the 
solar  structure.  At  the  best,  drawings  based  upon  visual 
observations  must  be  unsatisfactory,  since  even  the  skilled 
hands  of  Langley  could  not  secure  the  perfect  precision 
which  is  so  desirable.  It  accordingly  might  be  hoped  that 
here,  as  in  other  departments  of  solar  research,  photography 
would  afford  the  necessary  means  of  securing  results  unat- 
tainable by  the  eye.  Unfortunately,  however,  this  hope  has 
been  only  partially  realized,  as  a  brief  consideration  of  the 
best  results  in  this  field  will  show. 

It  is  a  comparatively  simple  matter  to  make  a  direct 
photograph  of  the  Sun.  It  is  only  necessary  to  form  a  solar 
image,  considerably  enlarged,  upon  a  "slow"  photographic 
plate,  and  then  to  give  an  excessively  short  exposure  by 
means  of  a  shutter  containing  a  narrow  slit,  which  is  shot 
across  just  in  front  of  the  plate  at  very  high  speed.  The  light 
from  any  part  of  the  Sun  reaches  the  plate  only  during  the 
brief  interval  in  which  the  slit  is  passing  the  corresponding 
part  of  the  image.  The  exposure  for  any  point  may  thus 
amount  to  no  more  than  rooVw  °^  a  second.  The  photo- 
graph reproduced  in  Plate  II  was  taken  in  this  way. 

In  order  to  obtain  photographs  showing  the  smaller 
details  of  the  photosphere,  it  is  desirable  to  use  a  solar 
image  enlarged  to  a  diameter  of  from  15  to  30  inches,  with 
photographic  plates  particularly  adapted  for  this  class  of  work. 
The  best  direct  photographs  hitherto  made  are  those  taken 
by  Janssen  at  the  Observatory  of  Meudon,  near  Paris.  A 
portion  of  one  of  these  pictures,  representing  the  great  Sun- 
spot  of  June  22,  1885,  is  reproduced  in  Plate  XXVII.  The 
penumbra  is  not  very  well  shown,  since  the  exposure  required 
for  the  brighter  regions  of  the  surrounding  photosphere  is 
too  short  to  bring  out  its  fainter  details.  Even  with  sufficient 
exposure,  however,  such  photographs  do  not  reveal  the  more 


PHENOMENA  OF  THE  SUN'S  SURFACE  71 

delicate  details  recorded  in  Langley's  drawings.  But  they 
do  show,  with  considerable  success,  the  minute  structure  of 
the  photosphere,  as  Plate  XXVII  illustrates.  Here  may  be 
seen,  autographically  recorded,  the  photospheric  "grains" 
which  Langley  believed  to  be  the  extremities  of  long  fila- 
ments reaching  down  toward  the  interior  of  the  Sun.  Jans- 
sen  holds  a  different  view,  since  he  regards  the  bright  grains 
to  be  small  spherical  masses  of  luminous  vapor,  separated  by 
vacant  regions.  In  chap,  xi  it  will  be  shown  that  the  results 
of  recent  investigations  with  the  spectroheliograph  tend  to 
bear  out  Langley's  view. 

There  can  be  little  doubt  that  direct  photographs  of  the 
Sun,  showing  smaller  details  than  have  yet  been  registered, 
will  ultimately  be  obtained.  Janssen's  photographs  have  all 
been  secured  with  a  small  instrument,  used  in  an  atmosphere 
where  the  conditions  are  not  particularly  favorable  for  work 
of  this  character.  It  is  therefore  to  be  hoped  that,  with  much 
more  powerful  apparatus,  employed  in  a  better  atmosphere, 
the  results  would  be  still  more  satisfactory. 

In  spite  of  long  study  and  much  discussion,  it  remains 
uncertain  whether  sun-spots  are  to  be  regarded  as  cavities  or 
as  elevated  regions  of  the  photosphere.  At  one  time  it  was 
supposed,  mainly  as  the  result  of  observations  made  by 
Wilson,  of  Glasgow,  in  the  eighteenth  century,  that  sun-spots 
were  saucer-shaped  cavities,  the  penumbra  representing  the 
sloping  edge  of  the  saucer,  with  the  umbra  at  the  center. 
More  recent  investigations,  however,  have  failed  to  confirm 
Wilson's  observations,  though  there  can  be  little  doubt  that 
the  umbra  lies  below  the  level  of  the  faculae  that  usually  sur- 
round spots.  Faculae  are  elevated  regions  of  the  photo- 
sphere, and  the  question  remains  open  whether  the  level  of  the 
umbra  is  above  or  below  the  average  level  of  the  photosphere, 
outside  of  the  faculae. 

The  only  other  phenomena  visible  in  direct  observations 


72  STELLAR  EVOLUTION 

of  the  Sun  are  the  faculae.  They  are  usually  most  numerous 
in  the  vicinity  of  Sun-spots,  and  near  the  Sun's  limb  they 
are  sometimes  very  conspicuous  brilliant  objects,  covering 
large  areas.  Near  the  center  of  the  Sun,  however,  they 
are  practically  invisible,  though  faint  traces  of  them  can 
sometimes  be  made  out  on  photographs  taken  with  a  suitable 
exposure.  This  increase  of  brightness  toward  the  Sun's  limb 
is  assumed  to  be  due  to  the  elevation  of  the  faculae  above  the 
photospheric  level,  and  their  escape  from  a  considerable  part 
of  that  absorption  which  so  materially  reduces  the  brightness 
of  the  photosphere.  Kising  above  the  denser  part  of  the 
absorbing  veil,  and  thus  suffering  but  little  diminution  of 
light,  they  appear  near  the  Sun's  limb  as  bright  objects 
on  a  less  luminous  background. 

Janssen's  photographs  tend  to  bear  out  the  assumption 
that  the  faculae  resemble  the  rest  of  the  photosphere,  differ- 
ing mainly  in  their  greater  altitude.  They  are  shown  by 
these  photographs  to  be  resolved  into  granular  elements 
similar  to  those  that  constitute  the  photosphere.  It  will  be 
seen  later,  however,  that  the  faculae  play  a  very  important 
r6le,  since  they  are  the  regions  from  which  immense  masses  of 
vapors  rise  to  the  solar  surface.  These  vapors  are  invisible 
to  the  eye,  and  no  trace  of  them  is  shown  on  photographs 
taken  in  the  manner  described  above.  But  they  may  be 
photographed  with  the  spectroheliograph,  by  the  method 
explained  in  chap.  xi. 


CHAPTER  X 

THE  SUN'S  SURROUNDINGS 

THE  first  observations  of  the  Sun's  surroundings  date  back 
to  an  early  period.  On  the  occasion  of  a  total  eclipse  the 
dark  body  of  the  Moon  covers  the  solar  disk,  cuts  off  the 
sunlight  which  at  other  times  illuminates  our  atmosphere, 
and  reveals  phenomena  ordinarily  hidden  by  its  glare.  It  is 
well  known  that,  if  our  atmosphere  were  absent,  there  would 
be  no  such  scattering  of  the  sunlight,  and  the  sky  would  be 
as  dark  during  the  day  as  it  now  appears  at  night.  In  such 
a  case  the  stars  would  be  visible  by  day,  as  well  as  the  solar 
corona.  Formerly,  when  no  artificial  means  of  reducing  this 
brilliant  illumination  of  the  atmosphere  were  known,  all 
knowledge  of  celestial  phenomena  in  the  immediate  vicinity 
of  the  Sun  was  of  necessity  obtained  during  total  eclipses. 
The  solar  corona  was  thus  discovered,  and  likewise  the  red 
flames,  or  prominences,  which  do  not  extend  so  far  from  the 
Sun's  surface. 

The  corona  still  remains  a  mysterious  phenomenon,  since 
no  means  has  yet  been  discovered  of  observing  it  without  an 
eclipse.  .  Our  knowledge  is  thus  confined  to  the  results  of 
observations  made  during  the  very  brief  periods  when  the 
Moon  shields  our  atmosphere  from  illumination  by  the  sun- 
light. The  general  appearance  of  the  corona,  as  seen  at 
the  eclipse  of  May  28,  1900,  is  illustrated  in  Plate  IV, 
reproduced  from  a  photograph  made  by  Barnard  and 
Ritchey.  It  may  be  described  as  a  faintly  luminous  veil 
of  light,  extending  outward  in  long  streamers  from  the  sur- 
face of  the  photosphere  to  distances  of  several  millions  of 
miles,  and  exceeded  in  brilliancy,  even  in  its  brightest 

73 


74  STELLAR  EVOLUTION 

parts,  by  the  full  Moon.  In  many  ways  its  streamers  re- 
semble those  of  the  aurora  borealis,  and  it  is  indeed  possible 
that  their  origin  may  be  ascribed  to  some  similar  electrical 
cause.  During  the  few  minutes  of  a  total  eclipse  they  are 
not  seen  to  undergo  change  of  form,  but  the  outline  of  the 
corona  does  vary  greatly  from  year  to  year,  in  sympathy 
with  the  general  variation  of  the  solar  activity  described  in 
another  chapter.  At  times  of  minimum  sun-spots  the  form  of 
the  corona  resembles  that  shown  in  Plate  IV.  This  minimum 
type  is  marked  by  great  winglike  extensions  along  the  solar 
equator,  and  by  much  shorter  streamers,  diverging  like  fans 
near  the  Sun's  pole.  At  times  of  maximum  sun-spots  the 
corona  is  much  more  extensive  in  the  polar  regions,  the 
streamers  equaling  in  length  those  of  the  equatorial  zone. 

Spectroscopic  observations  have  shown  that  the  corona  con- 
sists mainly  of  gases  unknown  to  the  chemist.  That  is  to 
say,  the  lines  in  its  spectrum  do  not  coincide  in  position  with 
the  lines  of  any  terrestrial  element.  Whether  these  gases, 
which  are  probably  very  light,  will  ultimately  be  found  on 
the  Earth  cannot  be  predicted.  Like  helium,  first  known  in 
the  Sun,  they  may  eventually  be  encountered,  in  minute 
quantities,  in  some  mineral,  where  they  have  hitherto  escaped 
the  chemist's  analysis.  The  fact  that  the  lower  part  of  the 
corona  gives  a  continuous  spectrum,  with  a  feeble  solar  spec- 
trum superposed  upon  it,  indicates  that  minute  incandescent 
particles  are  present,  which  are  hot  enough  to  radiate  white 
light,  and  which  scatter  enough  sunlight  to  account  for  the 
presence  of  the  solar  spectrum. 

It  may  now  be  of  interest  to  explain  how  the  solar  corona 
is  photographed  during  a  total  eclipse  of  the  Sun,  especially 
as  the  same  means  are  employed  during  eclipses  in  photo- 
graphing the  solar  prominences,  and  also  because  we  shall 
have  occasion  in  a  subsequent  chapter  to  refer  more  at 
length  to  the  general  type  of  telescope  here  represented. 


THE  SUN'S  SURROUNDINGS  75 

It  has  already  been  explained  that  the  size  of  an  image  of 
the  Sun  given  by  a  telescope  depends  directly  upon  the  focal 
length  of  the  lens  employed.  In  order  to  show  as  distinctly 
as  possible  the  more  minute  phenomena  of  the  corona,  it  is 
therefore  desirable  to  obtain  large-scale  photographs  of  it 
with  a  telescope  of  great  focal  length.  Obviously  such  an 
instrument  as  the  40-inch  Yerkes  refractor  could  not  easily 
be  transported  to  the  more  or  less  remote  regions  of  the 
Earth  where  the  passing  shadow  of  the  Moon  may  render  a 
total  eclipse  visible.  Fortunately,  however,  the  size  of  the 
focal  image  does  not  depend  upon  the  diameter  of  a  lens,  but 
merely  upon  its  focal  length.  Hence  the  desired  result  can 
be  obtained  by  using  a  long-focus  lens  of  comparatively 
small  diameter.  In  some  cases  such  lenses  are  pointed 
directly  at  the  Sun,  and  the  motion  of  the  solar  image, 
caused  by  the  Earth's  rotation,  is  compensated  for  by  a  cor- 
responding motion  of  the  photographic  plate  on  which  the 
image  falls.  Another  method,  which  offers  various  points  of 
advantage,  is  illustrated  in  Plate  XXVIII,  reproduced  from 
a  photograph  of  the  horizontal  telescope  used  by  the  eclipse 
party  of  the  Yerkes  Observatory  at  Wadesboro,  North  Caro- 
lina, on  May  28,  1900. 

The  essential  feature  of  this  instrument  is  a  plane  mirror, 
12  inches  in  diameter,  which  reflects  the  Sun's  rays  hori- 
zontally through  a  long  tube.  The  plane  of  the  mirror  is 
parallel  to  the  Earth's  axis,  and,  by  means  of  an  accurate 
driving-clock,  the  mirror  is  made  to  complete  a  rotation  once 
in  48  hours.  Such  a  motion  of  the  mirror  is  just  sufficient  to 
counteract  the  effect  of  the  Earth's  rotation,  and  thus  to  keep 
the  Sun's  rays  reflected  in  the  same  direction  for  an  indefinite 
period.  After  leaving  the  coelostat  mirror,  the  rays  fall  upon 
a  6-inch  photographic  lens,  which  forms  an  image  upon  a 
sensitive  plate  at  its  focus,  61^  feet  away.  Through  the 
rotation  of  the  mirror  the  image  is  maintained  at  a  fixed 


76  STELLAK  EVOLUTION 

position  upon  the  photographic  plate,  so  that  any  desired 
exposvire  may  be  given. 

With  this  apparatus  some  remarkably  fine  photographs  of 
the  corona  and  prominences  were  obtained  by  Barnard  and 
Ritchey,  of  the  Yerkes  Observatory  party  (Plate  XXIX). 
During  the  87  seconds  of  the  eclipse  seven  exposures  were 
made,  ranging  in  length  from  ^  second  to  30  seconds.  Several 
of  the  photographic  plates  used  were  25  X  30  inches  in  size. 
To  facilitate  rapid  handling,  they  were  mounted  on  a  wooden 
carrier  15  feet  long,  free  to  move  on  ball  bearings  on  steel 
rails  extending  at  right  angles  to  the  tube  through  the  en- 
tire length  of  the  photographic  house.  A  catch,  operated  by 
hand,  served  to  stop  the  plate-carrier  at  the  proper  position 
for  each  exposure. 

The  solar  prominences  are  seen  at  total  eclipses  of  the 
Sun,  projecting  like  red  flames  beyond  the  dark  edge  of  the 
Moon  (Plate  XXIX).  With  our  present  knowledge  of  these 
phenomena,  it  seems  hardly  possible  that  just  prior  to  the 
middle  of  the  last  century  they  were  regarded  by  some 
observers  as  the  illuminated  summits  of  lunar  mountains. 
Their  truly  solar  origin  was  conclusively  demonstrated  in 
1860,  when  they  were  photographed  by  Secchi  and  de  la 
Rue,  and  were  shown  not  to  share  the  motion  of  the  Moon. 
At  that  time,  however,  no  conclusions  could  be  drawn  as  to 
their  chemical  composition,  and  it  was  not  until  1868  that 
their  gaseous  nature  and  their  connection  with  the  Sun 
became  known  through  the  use  of  the  spectroscope.  It 
was  then  found  that  these  immense  masses  of  hydrogen  and 
helium  gas  rise  from  a  sea  of  flame  (the  chromosphere,  which 
completely  envelops  the  Sun),  and  sometimes  attain  eleva- 
tions of  hundreds  of  thousands  of  miles. 

The  rarity  and  brief  duration  of  total  eclipses  would  have 
limited  greatly  our  knowledge  of  the  prominences,  had  not 
Janssen,  Lockyer,  and  Huggins  devised  an  epoch-making 


THE  SUN'S  SURROUNDINGS 


77 


method  by  which  they  can  be  observed  on  any  clear  day,  in 
spite  of  the  glare  of  our  atmosphere  near  the  Sun.  The 
instrument  which  permits  this  result  to  be  accomplished  is 
the  spectroscope,  used 
in  conjunction  with  a 
telescope.  The  prin- 
ciple of  the  method  is 
simple  and  easily  un- 
derstood. The  white 
light  of  the  sky,  when 
passed  through  the 
spectroscope,  is  drawn 
out  into  a  long  rain- 
bow band,  and  there- 
by greatly  reduced  in  ^  /.^  o 
intensity.  The  light 
of  the  prominences, 
on  the  contrary,  is 
concentrated  in  the  radiations  characteristic  of  hydrogen  and 
helium  gas,  and  the  dispersing  power  of  the  spectroscope 
merely  separates  more  and  more  widely  the  colored  images 
which  correspond  to  these  radiations,  without  seriously  en- 
feebling them.  With  the  spectroscope  they  therefore  become 
visible,  since  their  images  are  brighter  than  the  highly  dis- 
persed background  of  skylight  on  which  they  lie. 

Plate  XXX  shows  a  solar  spectroscope  suitable  for  ob- 
serving the  spectra  or  the  forms  of  the  prominences  in  full 
sunlight.1  This  spectroscope  consists  essentially  of  a  slit 
(s  in  the  accompanying  diagram,  Fig.  4),  which  may  be  set 
tangentially  or  radially  upon  the  Sun's  limb;  a  collimating 
lens,  /,  which  renders  parallel  the  rays  coming  to  it  from 
the  slit;  a  plane  grating,  g,  ruled  with  about  15,000  lines  to 

1  This  spectroscope,  here  shown  attached  to  the  Yerkes  telescope,  was  formerly 
used  as  a  spectroheliograph  with  the  Kenwood  telescope  (Plate  XXXIV). 


FIG  4 
Diagram  of  Solar  Spectroscope 


78  STELLAR  EVOLUTION 

the  inch;  and  a  second  lens  and  eye-piece,  t  and  e,  which 
form  the  observing  telescope.  The  grating  diffracts  the  light 
which  reaches  it  from  the  collimating  lens,  and  produces  a 
spectrum,  an  image  of  which  is  formed  by  the  lens  /,  in  the 
focal  plane  of  the  eye-piece  e.  If  it  is  desired  to  photograph 
the  spectrum,  the  eye-piece  may  be  replaced  by  a  sensitive 
plate. 

If  we  wish,  for  example,  to  observe  the  spectrum  of  the 
chromosphere  with  this  instrument,  the  slit,  about  1/1,000 
of  an  inch  wide,  is  made  exactly  tangential  to  the  solar  image. 
Under  these  circumstances  the  observer  at  the  instrument 
will  see  the  spectrum  of  the  bright  sky  near  the  Sun,  which 
is  of  course  merely  the  spectrum  of  reflected  sunlight,  and  is 
therefore  crossed  by  all  of  the  dark  Fraunhofer  lines.  In  the 
case  of  substances  which  are  present  in  the  chromosphere,  the 
lines  of  the  spectrum  will  be  reversed  from  dark  to  bright  in 
regions  which  correspond  to  the  section  of  the  chromosphere 
lying  upon  the  slit.  The  most  conspicuous  bright  lines  to  be 
observed  in  this  way  are  the  hydrogen  lines  Ha  (red),  H(S 
(blue-green),  Hj  (blue),  and  H$  (violet),  and  the  brilliant 
yellow  helium  line  D3. 

The  history  of  this  helium  line  affords  an  interesting 
illustration  of  the  intimacy  of  the  relationship  which  now 
unites  terrestrial  and  solar  chemistry.  In  his  first  observa- 
tions of  the  spectrum  of  the  prominences,  made  in  1868, 
Lockyer  was  attracted  by  the  presence  of  a  bright  line  in  the 
yellow,  not  far  from  the  position  of  the  D,  and  D2  lines  of 
sodium.  This  line  was  designated  as  D3,  but  all  attempts  to 
identify  it  among  the  lines  of  known  elements  were  unsuc- 
cessful. Accordingly,  it  was  assumed  to  represent  a  new  gas, 
probably  very  light,  on  account  of  its  association  with  hydro- 
gen at  great  elevations  above  the  solar  surface.  Lockyer 
gave  the  name  "helium"  to  this  gas,  because  of  its  solar 
origin.  In  1895  Kamsay,  while  engaged  in  an  analysis  of 


THE  SUN'S  SURROUNDINGS  79 

the  mineral  cleveite,  discovered  an  unknown  gas,  which  he 
found  to  give  a  yellow  line  near  the  position  of  D3.  The 
spectroscope  he  employed  was  not  powerful  enough  to  deter- 
mine the  position  of  the  line  with  great  accuracy,  but  Kunge 
proved  beyond  a  doubt,  a  short  time  later,  that  the  line  was 
actually  in  the  position  of  D3.  However,  he  detected  a  faint 
companion  to  this  line,  on  the  side  toward  the  red,  which 
had  never  been  observed  in  the  solar  prominences.  An  exam- 
ination of  D3  in  the  prominence  spectrum,  made  at  the  Ken- 
wood Observatory  immediately  upon  the  receipt  of  Runge's 
description  of  his  laboratory  results,  showed  the  undoubted 
presence  of  a  similar  companion,  which  was  found  by  repeated 
measures  to  agree  well  in  position  with  Runge's  determina- 
tions. The  companion  was  so  faint  that  it  would  easily  escape 
observations  made  without  knowledge  of  its  existence.  It 
may  be  said,  however,  that  this  first  observation  was  greatly 
facilitated  by  the  presence  of  a  very  bright  prominence,  in 
which  D3  was  beautifully  shown.  Huggins  and  others  de- 
tected the  duplicity  of  the  line  about  the  same  time. 

As  was  anticipated  by  its  behavior  in  the  Sun,  helium  was 
found  to  be  the  lightest  of  all  known  gases,  except  hydrogen. 
Further  study  of  the  spectrum  showed  D3  to  be  only  one  of 
a  series  of  lines,  other  members  of  which  are  also  represented 
in  the  chromosphere.  Many  lines  characteristic  of  the  spectra 
of  "Orion"  stars,  which  had  not  been  identified  before  Ram- 
say's discovery,  are  also  due  to  helium.  It  is  extremely  prob- 
able that  other  new  elements,  not  yet  discovered  on  the  Earth, 
are  represented  by  some  of  the  unknown  lines  of  the  solar 
spectrum. 

While  the  lines  of  hydrogen  and  helium  are  more  bril- 
liant and  conspicuous  than  all  others  in  the  visible  spectrum 
of  the  chromosphere,  it  is  nevertheless  true  that  a  very  large 
number  of  lines  due  to  other  elements  can  be  seen  on  any 
good  day  with  a  powerful  telescope  and  spectroscope.  The 


80  STELLAK  EVOLUTION 

vapors  of  magnesium,  iron,  and  several  other  substances  are 
conspicuously  represented  by  bright  lines  in  the  chromo- 
spheric  spectrum;  but  these  lines  are  shorter  than  those  of 
hydrogen  and  helium,  since  the  vapors  do  not  rise  to  so  great 
a  height.  With  the  Yerkes  telescope  it  is  even  possible  to 
observe  a  multitude  of  fine  bright  lines  due  to  the  vapor  of 
carbon,  which  lies  in  close  contact  with  the  photosphere. 
The  layer  of  carbon  vapor  is  so  thin  that  the  least  motion 
of  the  solar  image,  or  a  very  slight  disturbance  of  the  atmos- 
phere, are  sufficient  to  render  the  lines  invisible. 

A  total  eclipse  affords  a  most  favorable  opportunity  to 
determine  photographically  the  depths  of  the  several  layers. 
The  simplest  way  of  accomplishing  this  is  to  place  a  prism 
over  the  object-glass  of  a  telescope,  which  is  directed  toward 
the  Sun.  When,  at  the  moment  of  totality,  the  Moon  covers 
the  photosphere,  arcs  of  the  chromosphere  are  left  projecting 
beyond  the  Moon's  edge.  After  passing  through  the  prism, 
the  image  formed  on  the  photographic  plate  will  appear  like 
that  reproduced  in  Plate  XXXI,  which  was  taken  by  Lord  at 
the  eclipse  of  1900.  The  arcs  represented  here  correspond 
to  the  various  lines  in  the  spectrum  of  the  chromosphere. 
In  this  case,  however,  since  no  slit  was  used  in  the  spectro- 
scope, the  form  of  each  arc  is  defined  by  the  distribution  of 
the  corresponding  vapor.  If  a  prominence  is  present  at  any 
point,  its  image  will  be  repeated  in  each  of  the  arcs  repre- 
senting the  element  it  contains.1  Of  course,  this  "spectrum 
of  the  flash,"  first  observed  by  Young,  and  so  called  on  account 
of  its  brief  duration,  can  be  photographed  only  during  the  few 
seconds  while  the  Moon's  edge  is  passing  over  the  chromo- 
sphere. 

It  will  now  be  seen  more  clearly  how  the  forms  as  well  as 
the  spectra  of  the  prominences  can  be  observed  by  the  spec- 

iThe  prominence  group  shown  in  Plate  XXIX  is  faintly  represented  here 
(reversed  in  position)  in  each  of  the  two  stronger  arcs. 


THE  SUN'S  SURROUNDINGS  81 

troscopic  method  without  an  eclipse.  So  long  as  a  narrow 
slit  is  employed,  the  spectrum  will  consist  of  narrow  lines, 
having  the  same  form  as  the  slit.  That  is,  if  the  slit  be 
straight,  the  lines  will  be  short,  straight  sections  of  the 
chromosphere  or  prominences,  corresponding  in  width  to  the 
slit.  If  the  slit  be  curved,  the  lines  will  have  a  corresponding 
curvature.  In  other  words,  the  lines  are  simply  monochro- 
matic images  of  the  slit.  Hence,  if  the  slit  be  widely  opened, 
the  lines  will  assume  the  form  of  that  portion  of  the  chromo- 
sphere or  prominence  which  happens  to  lie  across  it.  It  is 
as  though  one  were  looking  out  through  a  narrow  window 
upon  a  mass  of  great  flames. 

The  application  of  the  spectroscopic  method  to  the  study 
of  the  chromosphere  and  prominences  marked  a  new  era  in 
solar  research.  Daily  observations  were  inaugurated  with 
great  enthusiasm  by  Lockyer,  Young,  Janssen,  and  other 
astronomers  in  Europe  and  America.  It  was  found  that  the 
prominences  could  be  divided  into  two  classes — quiescent, 
or  cloudlike,  and  eruptive.  The  former  are  much  the  more 
numerous,  and  may  always  be  seen,  in  larger  or  smaller  num- 
bers, at  the  Sun's  limb.  They  change  slowly  in  form,  and 
sometimes  persist  for  days,  or  until  carried  out  of  view  by 
the  solar  rotation.  When  seen  under  excellent  atmospheric 
conditions,  the  complex  details  of  their  structure  resemble 
those  of  the  clouds  in  our  own  atmosphere.  The  eruptive 
prominences  change  very  rapidly  in  appearance,  sometimes 
shooting  up  to  elevations  of  over  two  hundred  thousand  miles 
in  a  few  minutes  (see  Plates  XXXV  and  XXXVI) .  Like  the 
quiescent  forms,  they  are  most  numerous  at  times  of  greatest 
sun-spot  activity.  They  are  never  observed  in  very  high 
latitudes,  though  the  quiescent  prominences  appear  at  all 
parts  of  the  solar  circumference.  The  photographic  study 
of  these  phenomena  will  be  described  in  the  next  chapter. 


CHAPTER  XI 
THE  SPECTROHELIOGRAPH 

THE  spectroscopic  method,  as  applied  by  astrophysicists 
in  various  parts  of  the  world,  has  yielded  a  nearly  continuous 
record  of  the  solar  prominences  extending  back  over  more 
than  thirty  years.  For  many  purposes  such  a  record  is 
entirely  satisfactory,  and  permits  important  conclusions  to 
be  drawn.  But  the  process  of  observation  is  not  only  slow 
and  painstaking :  it  is  subject  to  the  errors  and  uncertainties 
that  necessarily  attend  the  hand  delineation  of  any  object, 
seen  through  a  fluctuating  atmosphere.  Moreover,  changes 
in  the  forms  of  eruptive  prominences  are  frequently  so  rapid 
that  the  draughtsman  cannot  record  them.  It  was  principally 
in  the  hope  of  simplifying  the  process  of  observation,  and  of 
rendering  it  more  rapid  and  more  accurate,  that  the  spectro- 
heliograph  was  devised  at  the  Kenwood  Observatory  in 
1889.1 

The  principle  of  this  instrument  is  very  simple.  Its 
object  is  to  build  up  on  a  photographic  plate  a  picture  of 
the  solar  flames,  by  recording  side  by  side  images  of  the 
bright  spectral  lines  which  characterize  the  luminous  gases. 
In  the  first  place,  an  image  of  the  Sun  is  formed  by  a  tele- 
scope on  the  slit  of  a  spectroscope.  The  light  of  the  Sun,  after 
transmission  through  the  spectroscope,  is  spread  out  into  a 
long  band  of  color,  crossed  by  lines  representing  the  various 
elements.  At  points  where  the  slit  of  the  spectroscope  hap- 
pens to  intersect  a  gaseous  prominence,  the  bright  lines  of 
hydrogen  and  helium  may  be  seen  extending  from  the  base 

1  It  was  subsequently  learned  that  the  method  embodied  in  the  spectrohelio- 
graph  had  been  suggested  by  Janssen  as  early  as  1869,  reinvented  by  Braun  of 
Kalocsa,  and  actually  tried  by  Lohse  at  Potsdam.  But  it  had  not  proved  a  success. 

82 


THE  SPECTBOHELIOGRAPH  83 

of  the  prominence  to  its  outer  boundary.  If  a  series  of  ach 
lines,  corresponding  to  different  positions  of  the  slit  on  the 
image  of  the  prominence,  were  registered  side  by  side  onti 
photographic  plate,  it  is  obvious  that  they  would  give  a  rep- 
resentation of  the  form  of  the  prominence  itself.  To  accom- 
plish this  result,  it  is  necessary  to  cause  the  solar  image  to 
move  at  a  uniform  rate  across  the  first  slit  of  the  spectro- 
scope, and,  with  the  aid  of  a  second  slit  (which  occupies  the 
place  of  the  ordinary  eyepiece  of  the  spectroscope) ,  to  isolate 
one  of  the  lines,  permitting  the  light  from  this  line,  and 
from  no  other  portion  of  the  spectrum,  to  pass  through  the 
second  slit  to  a  photographic  plate.  If  the  plate  be  moved 
at  the  same  speed  with  which  the  solar  image  passes  across 
the  first  slit,  an  image  of  the  prominence  will  be  recorded 
upon  it.  The  principle  of  the  instrument  thus  .lies  in  photo- 
graphing the  prominence  through  a  narrow  slit,  from  which 
all  light  is  excluded  except  that  which  is  characteristic  of  the 
prominence  itself.  It  is  evidently  immaterial  whether  the 
solar  image  and  photographic  plate  are  moved  with  respect 
to  the  spectroheliograph  slits,  or  the  slits  with  respect  to  a 
fixed  solar  image  and  plate. 

This  method,  when  first  tried  at  the  Harvard  Observa- 
tory in  1890,  proved  unsuccessful.  The  lack  of  success 
was  partly  due  to  the  fact  that  a  line  of  hydrogen  was 
employed.  This  'line,  though  fairly  suitable  for  the  pho- 
tography of  prominences  with  the  perfected  spectrohelio- 
graph of  the  present  day,  was  too  faint  for  successful  use 
amidst  the  difficulties  which  surrounded  the  first  experi- 
ments. Accordingly,  when  the  work  was  resumed  a  year 
later  at  the  Kenwood  Observatory  in  Chicago  (Plate  XXXIII) 
an  attempt  was  made,  through  a  photographic  investigation 
of  the  violet  and  ultra-violet  regions  of  the  prominence  spec- 
trum, to  discover  other  lines  better  fitted  for  future  experi- 
ments. In  the  extreme  violet  region,  in  the  midst  of  two 


84  STELLAR  EVOLUTION 

broad  dark  bands  which  form  the  most  striking  feature  of  the 
solar  spectrum,  two  bright  lines  (H  and  K)  were  found  and 
attributed  to  the  vapor  of  calcium.  They  had  previously 
been  seen  visually  by  Young,  but,  on  account  of  the  insensi- 
tiveness  of  the  eye  for  light  of  this  color,  they  could  not  be 
observed  satisfactorily.  A  careful  study  soon  showed  them 
to  be  present  in  every  prominence  examined,  at  elevations 
above  the  solar  surface  equaling  or  exceeding  those  attained 
by  hydrogen  itself  (Plate  XXXII,  a).  Their  suitability  for 
the  purpose  of  prominence  photography  is  due  to  several 
causes,  among  which  may  be  mentioned  their  exceptional 
brilliancy,  their  presence  at  the  center  of  broad  dark  bands 
which  greatly  diminish  the  brightness  of  the  sky  spectrum, 
and  the  comparatively  high  sensitiveness  of  photographic 
plates  for  light  of  this  wave-length. 

While  fairly  efficient  from  an  optical  point  of  view,  the 
spectroheliograph  of  the  preceding  year  had  possessed  many 
mechanical  defects.  It  sufficed  to  give  photographs  of 
individual  prominences,  but  they  were  not  very  satisfactory. 
In  a  new  instrument,  devised  for  use  with  the  12-inch  Ken- 
wood telescope,  the  principal  defects  were  overcome,  and 
means  of  securing  the  necessary  conditions  of  the  experi- 
ment were  provided.  The  Kenwood  spectroheliograph  is 
shown  in  Plate  XXXIV.  In  this  instrument  the  solar  image 
and  photographic  plate  were  fixed,  while  the  first  and  second 
slits  were  made  to  move  across  them  by  means  of  a  system 
of  levers,  set  in  motion  by  hydraulic  power.  The  first  trials 
of  the  instrument,  made  in  January,  1892,  were  entirely 
successful,  and  the  chromosphere  and  prominences  surround- 
ing the  Sun's  disk  were  easily  and  rapidly  recorded  (Plates 
III,  XXXV,  and  XXXVI).  The  details  of  their  structure 
were  shown  with  the  sharpness  and  precision  characteristic 
of  the  best  eclipse  photographs.  And  the  opportunity  for 
making  such  records,  previously  limited  to  the  brief  dura- 


THE  SPECTROHELIOGRAPH  85 

tion,  never  exceeding  seven  minutes,  of  a  total  eclipse,  was 
at  once  indefinitely  extended.  Thus  it  became  possible  to 
study  photographically  the  slowly  varying  forms  of  the  qui- 
escent, cloudlike  prominences,  and,  to  particular  advantage, 
the  rapid  changes  of  violent  eruptions. 

But  even  before  this  primary  purpose  of  the  work  had 
been  accomplished,  the  possibility  of  making  another  and 
much  more  important  application  of  the  instrument  had 
presented  itself.  A  photographic  study  of  the  spectrum  of 
various  portions  of  the  Sun's  surface  had  shown  the  existence 
at  many  points  of  great  clouds  of  calcium  vapor,  luminous 
enough  to  render  their  existence  evident  through  the  produc- 
tion of  bright  H  and  K  lines  on  the  solar  disk  (Plate  XXXII, 
b  and  c).  Some  of  these  calcium  clouds  had,  indeed,  been 
known  to  exist  through  the  important  visual  observations  of 
Young,  who  had  observed  the  bright  H  and  K  lines  in  the 
vicinity  of  sun-spots.  But  the  vast  extent  and  the  charac- 
teristic forms  of  the  phenomena  could  not  be  ascertained 
'by  such  means.  What  was  required  was  such  a  repre- 
sentation of  the  solar  disk  as  the  spectroheliograph  had  been 
designed  to  give  in  the  case  of  the  prominences.  From  a 
consideration  of  the  results  obtained  in  the  spectroscopic 
study  of  the  disk,  it  appeared  probable  that  an  important 
application  of  the  spectroheliograph  might  be  made  in  this 
new  direction. 

Before  describing  this  second  application  of  the  instru- 
ment, it  may  be  well  to  recall  the  appearance  of  the  Sun 
when  seen  with  a  telescope,  or  when  photographed  in  the 
ordinary  manner  without  a  spectroheliograph.  From  photo- 
graphs like  that  reproduced  in  Plate  II,  we  see  that  the 
most  conspicuous  features  of  the  solar  surface,  at  least  so  far 
as  the  eye  can  detect,  are  the  well-known  sun-spots.  The 
bright  faculae,  which  rise  above  the  photosphere,  are  con- 
spicuous when  near  the  edge  of  the  Sun,  but  practically 


86  STELLAR  EVOLUTION 

invisible  when  they  happen  to  lie  near  the  center  of  the  disk. 
The  bright  H  and  K  lines,  referred  to  in  the  last  paragraph, 
were  found  in  close  association  with  the  faculae,  and  it 
appeared  probable  that  much  of  the  highly  heated  calcium 
vapor,  to  which  these  bright  lines  are  due,  rises  from  the 
interior  of  the  Sun  through  the  faculae.  It  was  therefore 
to  be  expected  that  a  successful  application  of  the  spectro- 
heliograph  to  the  photography  of  the  luminous  calcium 
clouds  would  give  bright  forms  resembling  those  of  the 
faculae.  Furthermore,  it  was  to  be  hoped  that  these  brilliant 
clouds  could  be  recorded,  not  only  near  the  limb  of  the  Sun, 
but  also  in  the  central  part  of  the  disk,  since  the  bright 
reversals  of  the  H  and  K  lines  were  equally  well  photo- 
graphed in  all  parts  of  the  image. 

The  results  of  the  first  experiments,  which  were  made  at 
the  beginning  of  1892,  were  such  as  to  justify  fully  the 
expectations  that  had  been  entertained.  It  was  at  once  found 
possible  to  record  the  forms,  not  only  of  the  brilliant  clouds 
of  calcium  vapor  associated  with  the  faculae,  and  occurring 
in  the  vicinity  of  sun-spots,  but  also  of  a  reticulated  struc- 
ture extending  over  the  entire  surface  of  the  Sun.  The 
earliest  use  of  the  method  was  made  in  the  study  of  the 
great  sun-spot  of  February,  1892,  which,  through  the  great 
scale  of  the  phenomena  it  exhibited  and  the  rapid  changes 
that  resulted  from  its  exceptional  activity,  afforded  the  very 
conditions  required  to  bring  out  the  peculiar  advantages  of 
the  spectroheliograph.  In  the  systematic  use  of  the  instru- 
ment continued  at  the  Kenwood  Observatory  through  the 
following  years,  a  great  variety  of  solar  phenomena  were 
recorded,  and  the  changes  which  they  underwent  from  day 
to  day — sometimes,  in  the  more  violent  eruptions,  from 
minute  to  minute — were  registered  in  permanent  form. 
During  this  period,  which  ended  with  the  transfer  of  the 
Kenwood  instruments  to  the  Yerkes  Observatory,  over  3,000 


THE  SPECTROHELIOGRAPH  87 

photographs  of  solar  phenomena  were  secured.  From  a 
systematic  study  of  these  negatives,  in  the  course  of  which 
the  heliographic  latitude  and  longitude  of  the  calcium  clouds 
(subsequently  named  the  flocculi)  in  many  parts  of  the  Sun's 
disk  were  measured  from  day  to  day  (by  Fox),  a  new  deter- 
mination of  the  rate  of  the  solar  rotation  in  various  latitudes 
has  been  made.  This  shows  that  the  calcium  flocculi,  like 
the  sun-spots,  complete  a  rotation  in  much  shorter  time  at  the 
solar  equator  than  at  points  nearer  the  poles.  In  other 
words,  the  Sun  does  not  rotate  as  a  solid  body  would  do,  but 
rather  like  a  ball  of  vapor,  subject  to  laws  which  are  not  yet 
understood. 

In  this  first  period  of  its  career  the  spectroheliograph 
had  therefore  permitted  the  accomplishment  of  two  principal 
objects.  It  had  provided  a  simple  and  accurate  means  of 
photographing  the  solar  prominences  in  full  sunlight,  which 
gave  results  hardly  inferior  to  those  obtained  during  the 
brief  moments  of  a  total  eclipse.  It  had  also  given  a  means 
of  recording  a  new  class  of  phenomena,  known  previously  to 
exist  only  through  glimpses  of  the  bright  calcium  lines  in  the 
vicinity  of  sun-spots,  but  wholly  invisible  to  observation, 
either  visually  or  on  photographs  taken  by  ordinary  methods. 
It  was  not  difficult  to  see,  however,  that  the  possibilities  of 
the  new  method  were  much  greater  than  had  been  indicated 
by  the  work  so  far  accomplished.  It  seemed  probable  that 
our  knowledge  of  the  finer  details  of  the  calcium  flocculi 
would  be  greatly  increased  if  provision  could  be  made  for 
photographing  a  much  larger  solar  image  with  a  spectro- 
heliograph of  improved  design.  And  it  was  furthermore 
evident  that  other  applications  of  the  instrument,  involving 
the  use  of  different  spectral  lines,  and  the  employment  of 
principles  which  had  not  been  thoroughly  tested  in  the 
earlier  work,  might  reasonably  be  hoped  for.  Attempts 
were,  indeed,  made  to  photograph  the  Sun's  disk  with  the 


STELLAK  EVOLUTION 


dark  lines  of  hydrogen,  but  the  Kenwood  spectroheliograph 
was  not  well  adapted  for  this  purpose. 

The  40-inch  telescope  of  the  Yerkes  Observatory  pro- 
vided the  first  requisite  for  the  new  work — namely,  a  large 
solar  image,  having  a  diameter  of  7  inches  as  compared  with 
the  2-inch  image  given  by  the  Kenwood  telescope.  The 
construction  of  a  spectroheliograph  large  enough  to  photo- 
graph such  an  image  of  the  Sun  involved  serious  difficulties, 
but  these  were  finally  overcome.  The  Rumford  spectro- 
heliograph, designed  to  meet  the  special  conditions  of  the 
new  work,  was  built  in  the  instrument  shop  of  the  Yerkes 
Observatory,  and  is  now  in  daily  use  with  the  40-inch 
telescope  (Plate  XXXVII). 

In  this  instrument  the  solar  image  is  caused  to  move 
across  the  first  slit  by  means  of  an  electric  motor,  which 
gives  the  entire  telescope  a  slow  and  uniform  motion  in 
declination.  The  sunlight,  after  passing  through  the  first 
slit,  is  rendered  parallel  by  a  large  lens  at  the  lower  end  of 
the  collimator  tube.  The  parallel  rays  from  this  lens  fall 
upon  a  silvered  glass  mirror,  from  which  they  are  reflected  to 
the  first  of  two  prisms,  by  which  they  are  dispersed  into  a 
spectrum  (Plate  XLI,  Fig.  1).  After  passing  through  the 
prisms,  the  light,  which  has  now  been  deflected  through  an 
angle  of  180°,  falls  upon  a  second  large  lens  at  the  lower 
end  of  the  camera  tube.  This  forms  an  image  of  the 
spectrum  at  the  upper  end  of  the  tube,  where  the  second 
slit  is  placed.  Any  line  in  the  spectrum  may  be  made  to 
fall  upon  this  slit,  by  properly  adjusting  the  mirror  and 
prisms.  Above  the  slit,  and  nearly  in  contact  with  it,  the 
photographic  plate  is  mounted  in  a  carriage,  which  runs  on 
rails  at  right  angles  to  the  length  of  the  slit.  The  rails  are 
covered  by  a  light-tight  camera  box,  so  that  no  light  can 
reach  the  plate  except  that  which  passes  through  the  second 
slit.  While  the  solar  image  is  moving  across  the  first  slit, 


THE  SPECTROHELIOGRAPH  89 

the  plate  is  moved  at  the  same  rate  across  the  second  slit,  by 
a  shaft  leading  down  the  tube  from  the  electric  motor,  and 
connected,  by  means  of  belting,  with  screws  that  drive  the 
plate-carriage. 

Photographs  of  the  solar  disk  taken  with  this  instrument 
under  good  atmospheric  conditions  reveal  a  multiplicity  of  fine 
details  (Plate  XXXVIII).  The  entire  surface  of  the  Sun  is 
shown  by  these  plates  to  be  covered  by  minute  luminous 
clouds  of  calcium  vapor,  only  about  a  second  of  arc  in 
diameter,  separated  by  darker  spaces,  and  closely  resembling 
in  appearance  the  well-known  granulation  of  the  solar  photo- 
sphere (Plate  XXXIX) .  A  sharp  distinction  must,  however, 
be  drawn  between  this  appearance,  which  is  wholly  invisible  to 
the  eye  at  the  telescope,  and  the  granulation  of  the  photosphere. 
In  accordance  with  Langley's  view,  the  grains  into  which  the 
solar  surface  is  resolved  under  good  conditions  of  visual  obser- 
vation are  the  extremities  of  columns  of  vapor  rising  from  the 
Sun's  interior.  They  seem  to  mark  the  regions  at  which 
convection  currents,  proceeding  from  within  the  Sun,  bring 
up  highly  heated  vapors  to  a  height  where  the  temperature 
becomes  low  enough  to  permit  them  to  condense.  It  might 
be  anticipated  that  out  of  the  summits  of  these  condensed 
columns,  other  vapors,  less  easily  condensed,  would  continue 
to  rise,  and  that  the  granulated  appearance  obtained  with 
the  spectroheliograph  may  represent  the  calcium  clouds 
thus  ascending  from  the  columns  (Plate  XL).  We  might, 
indeed,  go  a  step  farther,  and  imagine  the  larger  and  higher 
calcium  clouds  to  be  constituted  of  similar  vaporous  columns, 
passing  upward  through  the  chromosphere,  and  perhaps  at 
times  extending  out  into  the  prominences  themselves.  A 
means  of  research  now  to  be  described,  which  represents 
another  application  of  the  spectroheliograph,  involving  a 
new  principle,  seems  competent  to  throw  some  light  on  this 
question. 


90  STELLAK  EVOLUTION 

Mention  has  already  been  made  of  the  faculae,  which  are 
simply  regions  in  the  photosphere  that  rise  above  the  ordi- 
nary level.  Near  the  edge  of  the  Sun  their  summits  lie 
above  the  lower  and  denser  part  of  that  absorbing  atmos- 
phere which  so  greatly  reduces  the  Sun's  light  near  the 
limb,  and  in  this  region  the  faculae  may  be  seen  visually. 
At  times  they  may  be  traced  to  considerable  distances  from 
the  limb,  but  as  a  rule  they  are  inconspicuous  or  wholly 
invisible  toward  the  -  central  part  of  the  solar  disk.  The 
Kenwood  experiments  had  shown  that  the  calcium  vapor 
coincides  closely  in  form  and  position  with  the  faculae,  and 
hence  the  calcium  clouds  were  long  spoken  of  under  this 
name.  In  the  new  work  at  the  Yerkes  Observatory  the  dif- 
ferences between  the  calcium  clouds  and  the  underlying 
faculae  became  so  marked  that  a  distinctive  name  for  the 
vaporous  clouds  appeared  necessary.  They  were  therefore 
designated  flocculi,  a  name  chosen  without  reference  to  their 
particular  nature,  but  suggested  by  the  flocculent  appearance 
of  the  photographs. 

In  order  to  analyze  these  flocculi  and  to  determine  their 
true  structure,  a  method  was  desired  which  would  permit 
sections  of  them  at  different  heights  above  the  photosphere  to 
be  photographed.  Fortunately  there  is  a  simple  means  (first- 
described  by  Deslandres)  which  appears  to  accomplish  this 
apparently  difficult  object.  At  the  base  of  the  flocculi  the 
calcium  vapor,  just  rising  from  the  Sun's  interior,  is  com- 
paratively dense.  As  it  passes  upward  through  the  flocculi 
it  reaches  a  region  of  much  lower  pressure,  and  during  the 
ascent  it  might  be  expected  to  expand,  and  therefore  to 
become  less  dense.  Now  we  know  from  experiments  in  the 
laboratory  that  dense  calcium  vapor  produces  very  broad 
spectral  bands,  and  that,  as  the  density  of  the  vapor  is 
decreased,  these  bands  narrow  down  into  fine,  sharp  lines 
(Plate  XLI,  Fig.  2).  An  examination  of  the  solar  spectrum 


THE  SPECTKOHELIOGRAPH  91 

will  show  that  the  H  and  K  lines  of  calcium  give  evidence  of 
the  occurrence  of  this  substance  under  widely  different  densi- 
ties in  the  Sun.  The  broad  dark  bands,  which  for  convenience 
we  designate  Hl  and  Kn  are  due  to  the  low-lying,  dense 
calcium  vapor  (Plate  XXXII).  At  their  middle  points  (over 
flocculi)  are  seen  two  bright  lines,  which  are  much  narrower 
and  better  defined.  These  lines,  designated  H2  and  K2,  are 
the  ones  ordinarily  employed  in  photographing  the  flocculi 
with  the  spectroheliograph.  Superposed  upon  these  bright 
lines  are  still  narrower  dark  lines,  due  to  the  absorption  of 
cooler  calcium  vapor  at  higher  elevations  (H3,  K3).  It  will 
be  seen  that  the  evidence  of  the  existence  of  calcium  vapor 
at  various  densities  in  the  Sun  is  apparently  complete,  and 
that  we  may  here  find  a  way  of  photographing  the  vapor  at 
low  levels  without  admitting  to  the  photographic  plate  any 
light  that  comes  from  the  rarer  vapors  at  higher  levels.  It  is 
simply  necessary  to  set  the  second  slit  of  the  spectrohelio- 
graph near  the  edge  of  the  broad  H1  or  K:  bands,  in  order 
to  obtain  a  picture  showing  only  that  vapor  which  is  dense 
enough  to  produce  a  band  of  width  sufficient  to  reach  this 
position  of  the  slit.  No  light  from  the  rarer  vapors  above 
can  enter  the  second  slit  under  these  circumstances,  since 
they  are  incapable  of  producing  a  band  of  the  necessary 
width.1 

The  great  sun-spot  of  October,  1903,  afforded  an  oppor- 
tunity to  try  this  method  in  a  very  satisfactory  manner. 
Sections  of  the  calcium  vapor  in  the  neighborhood  of  this 
spot-group,  corresponding  to  the  two  different  levels  photo- 
graphed on  October  9,  are  shown  in  Figs.  1  and  2,  Plate 

1  The  bright  regions  photographed  in  this  way  resemble  the  f  aculae  very  closely, 
and  may  be  regarded  as  essentially  identical  with  them,  since  the  white  light  from 
the  continuous  spectrum  of  the  faculae  contributes  in  an  important  degree  to  the 
formation  of  the  photographic  images.  However,  any  dense  calcium  vapor  which 
extends  beyond  the  boundaries  of  the  faculae  will  be  recorded  on  the  photograph. 
In  any  case  we  should  expect  the  dense  calcium  vapor,  supposed  to  be  rising  from 
the  faculae,  to  correspond  closely  with  them  in  form. 


92  STELLAR  EVOLUTION 

XLII.1  The  manner  in  which  the  vapor  at  the  H2  level  over- 
hangs the  edge  of  the  sun-spot  is  very  striking,  and  thorough 
study  should  throw  some  light  on  the  conditions  which  exist 
in  such  regions.  For  it  is  possible,  not  only  to  photograph 
sections  of  the  vapor  at  various  levels,  but  also  to  ascertain, 
by  the  displacement  of  the  H2  or  H3  line,  as  photographed  by 
a  powerful  spectrograph,  the  direction  and  velocity  of  motion 
of  the  vapor  which  constitutes  the  flocculi.  It  is  commonly 
found  that  the  vapor  is  moving  upward  at  the  rate  of  about 
one  kilometer  per  second,  though  the  velocity  varies  con- 
siderably at  different  points  and  under  different  conditions. 
The  photographs  occasionally  show  the  existence  of  flocculi 
remarkable  for  their  great  brilliancy.  In  these  regions  active 
eruptions  are  in  progress.  The  vapor,  rendered  highly 
luminous  by  intense  heat  or  other  causes,  is  shot  out  from 
the  Sun's  interior  with  great  velocity.  Consequently  there 
are  rapid  changes  in  the  forms  of  these  brilliant  regions, 
whereas  the  ordinary  flocculi  change  slowly,  and  represent 
a  much  less  highly  disturbed  condition  of  affairs.  The 
brilliant  eruptive  flocculi  always  occur  in  active  regions  of 
the  solar  surface,  and  probably  correspond  with  the  erup- 
tive prominences  sometimes  photographed  projecting  from 
the  Sun's  limb.  A  remarkable  instance  was  recorded  on 
the  Kenwood  photographs,  which  showed  four  successive 
stages  of  an  eruption  of  calcium  vapor  on  an  enormous  scale. 
A  vast  cloud  thrown  out  from  the  Sun's  interior  completely 
blotted  from  view  a  large  sun-spot,  and  spread  out  in  a  few 
minutes  so  as  to  cover  an  area  of  four  hundred  millions  of 
square  miles. 

1  Although  these  photographs  have  been  arranged  for  comparison  with  the 
stereoscope,  it  is  to  be  understood  that  no  stereoscopic  effect  in  the  ordinary  sense 
will  be  obtained  in  examining  them.  The  purpose  of  using  the  stereoscope  is  simply 
to  allow  the  images  to  be  superposed,  thus  permitting  them  to  be  seen  at  the  same 
point  in  rapid  succession  by  moving  a  card  so  as  to  cover  alternately  the  two  lenses 
of  the  stereoscope.  Thus  the  manner  in  which  the  calcium  flocculi  overhang  the 
penumbra,  and  sometimes  the  umbra,  of  spots  can  be  observed. 


THE  SPECTROHELIOGRAPH  93 

Although  the  eruptive  flocculi  probably  correspond  in 
many  instances  with  eruptive  prominences,  it  must  not  be 
concluded  that  the  quiescent  calcium  flocculi  correspond 
with  the  quiescent,  cloudlike  prominences.  As  a  matter  of 
fact,  we  have  good  evidence  for  the  belief  that  the  flocculi 
shown  in  these  photographs  represent  in  most  instances 
comparatively  low-lying  vapors,  while  the  prominences, 
which  extend  above  the  level  of  the  chromosphere,  do  not 
ordinarily  reveal  themselves  as  bright  objects  in  projection 
against  the  disk. 

So  far,  we  have  considered  the  photography  of  the  Sun 
with  the  light  of  the  H  and  K  lines  of  calcium.  But  it 
must  naturally  occur  to  anyone  familiar  with  the  solar 
spectrum  that  it  should  be  possible  to  take  photographs 
corresponding  to  other  lines,  and  thus  representing  the 
vapors  of  other  substances.  For  the  darkness  of  the  lines 
is  only  relative;  if  they  could  be  seen  apart  from  the  bright 
background  of  continuous  spectrum  on  which  they  lie,  these 
lines  would  shine  with  great  brilliancy.  It  is  thus  evident 
that,  if  all  light  except  that  which  comes  from  one  of  these 
dark  lines  can  be  excluded  from  the  photographic  plate  by 
means  of  the  second  slit  of  the  spectroheliograph,  it  should  be 
possible  to  obtain  a  photograph  showing  the  distribution  of 
the  vapors  corresponding  to  the  line  in  question. 

At  this  point  attention  should  be  called  to  the  extreme 
sensitiveness  of  the  spectroheliograph  in  recording  minute 
variations  in  the  intensity  of  a  line — variations  so  slight  that 
no  trace  of  them  can  be  seen  in  a  spectrum  photograph 
showing  only  the  line  itself.  A  well-known  physiological 
effect  is  here  concerned,  for  it  is  common  experience  that 
the  eye  cannot  detect  minute  differences  of  intensity  in 
various  parts  of  an  extremely  narrow  line,  whereas  these 
would  become  conspicuous  if  the  line  were  widened  out 
into  a  band  of  considerable  width.  The  spectroheliograph 


94  STELLAK  EVOLUTION 

records  side  by  side  upon  the  photographic  plate  a  great 
number  of  images  of  a  line  which,  taken  together,  build  up 
the  form  of  the  region  from  which  the  light  proceeds.  In 
this  way  the  full  benefit  of  the  physiological  principle  is 
derived,  and  very  minute  differences  of  intensity  at  various 
parts  of  the  solar  disk  are  clearly  registered  upon  the 
plate. 

It  is  obviously  essential  in  photographing  with  the  dark 
lines  to  exclude  completely  the  light  from  the  continuous 
spectrum  on  either  side  of  the  line  employed.  The  admis- 
sion of  even  a  small  quantity  of  this  light  might  completely 
nullify  the  slight  differences  of  intensity  recorded  by  the 
aid  of  the  comparatively  faint  light  of  the  dark  line.  As 
the  second  slit  cannot  be  narrowed  beyond  a  certain  point, 
it  is  evident  that  for  successful  photography  with  the  dark 
lines  their  width  must  be  increased  by  dispersion  in  the 
spectroheliograph  to  such  a  degree  as  to  make  them  wider 
than  the  second  slit. 

The  first  satisfactory  photographs  obtained  with  dark  lines 
were  made  with  the  Rumford  spectroheliograph  in  May,  1903. 
The  lines  of  hydrogen  were  chosen  for  this  purpose,  on 
account  of  their  considerable  breadth,  and  because  of  the 
prominent  part  played  by  this  gas  in  the  chromosphere  and 
prominences.  In  order  to  secure  sufficient  width  of  the 
lines,  the  mirror  of  the  spectroheliograph  was  replaced  by  a 
large  plane  grating  having  20,000  lines  to  the  inch.  After 
leaving  the  grating  the  diffracted  light  enters  the  prisms, 
where  it  is  still  further  dispersed  before  the  image  of  the 
spectrum  is  formed  upon  the  second  slit.  The  effect  of  the 
prisms  is  not  only  to  give  additional  dispersion,  but  also  to 
reduce  the  intensity  of  the  diffuse  light  from  the  grating — 
a  most  important  matter  in  work  of  this  nature.  The  hydro- 
gen lines  employed  were  1Z/3,  Hy,  or  HS,  in  the  green-blue, 
blue,  and  violet,  respectively. 


THE  SPECTROHELIOGRAPH  95 

On  developing  the  first  plate  it  was  surprising  to  find 
evidences  of  a  mottled  structure  covering  the  Sun's  disk, 
resembling  in  a  general  way  the  structure  of  the  calcium 
flocculi,  but  differing  in  the  important  fact  that,  whereas  the 
calcium  flocculi  are  bright,  those  of  hydrogen  are  dark 
(Plate  XLIII).  This  result  was  confirmed  by  subsequent 
photographs,  and  it  was  found  that  in  general  the  hydrogen 
flocculi  are  dark,  although  in  certain  disturbed  regions  bright 
hydrogen  flocculi  appear.  Some  of  these  are  eruptive  in 
character,  and  correspond  closely  with  the  brilliant  eruptive 
calcium  flocculi.  But  in  other  cases,  in  regions  where  no 
violent  eruptive  disturbances  seem  to  be  present,  the  hydrogen 
flocculi  frequently  appear  bright  instead  of  dark  (Plate 
LXXII) .  Such  regions  are  usually  in  the  immediate  vicinity 
of  active  sun-spots,  where  it  is  probable  that  the  temperature 
of  the  hydrogen  is  considerably  higher  than  in  the  surround- 
ing regions.  Since  a  higher  temperature  would  undoubtedly 
produce  increased  brightness,  the  spectroheliograph  thus 
seems  to  afford  a  method  of  distinguishing  between  regions 
of  higher  and  lower  temperature — an  additional  property 
which  should  prove  of  great  value  in  investigations  on  the 
vapors  associated  with  sun-spots.  It  is  possible,  of  course, 
that  the  increased  brightness  is  due,  not  merely  to  an 
increase  of  temperature,  but  to  other  causes,  perhaps  of  a 
chemical  or  electrical  nature,  which  are  not  yet  understood. 
But  the  assumption  that  increased  temperature  is  the  effective 
cause  may  be  provisionally  accepted  as  very  probable. 

The  comparative  darkness  of  the  ordinary  hydrogen  flocculi 
evidently  indicates  that  this  gas  in  the  flocculi  for  some  rea- 
son radiates  less  light  than  the  hydrogen  gas  which,  probably 
after  diffusing  from  the  flocculi,  has  spread  in  a  nearly  uni- 
form mass  over  the  entire  surface  of  the  Sun.  The  simplest 
hypothesis  is  to  assume  that  the  diminished  brightness  of 
the  flocculi  is  due  to  the  reduced  temperature  in  the  upper 


96  STELLAK  EVOLUTION 

chromosphere,  where  the  absorption  probably  occurs.  The 
results  of  work  at  Mount  Wilson,  described  in  chap,  xvi,  seem 
to  render  this  view  probable.  It  should  be  emphasized  at 
this  point,  however,  that  the  explanation  of  spectroheliograph 
results  offered  in  this  chapter  is  merely  an  hypothesis,  which 
subsequent  investigation  may  not  prove  to  be  correct. 
According  to  Julius,  the  flocculi  are  not  luminous  clouds,  but 
the  effects  of  anomalous  dispersion  of  light  passing  out  from 
the  Sun's  interior  through  vapors  of  unequal  density  (see 
p.  148). 

The  Rumford  spectroheliograph  was  also  used  to  secure 
photographs  with  some  of  the  stronger  dark  lines  of  iron 
and  other  substances.  But  even  with  the  grating  the  disper- 
sion was  insufficient  to  give  thoroughly  trustworthy  results, 
except  in  a  very  few  cases.  It  was  evident  that  much  greater 
dispersion  must  be  employed  in  order  to  realize  the  full  advan- 
tages of  the  method  in  future  work.  Subsequent  progress 
in  the  development  of  the  spectroheliograph  is  described  in 
chap.  xvi. 

Within  a  short  time  after  the  first  work  at  the  Kenwood 
Observatory  the  spectroheliograph  came  into  general  use. 
Evershed  constructed  and  successfully  used  one  of  these 
instruments  in  England,  and  a  year  later  Deslandres,  whose 
admirable  work  on  the  spectra  of  the  flocculi  was  contempo- 
raneous with  the  investigations  at  the  Kenwood  Observatory, 
undertook  systematic  research  with  a  spectroheliograph  at 
the  Paris  Observatory.  His  contributions  to  the  develop- 
ment of  the  instrument  have  been  very  valuable.  Other 
spectroheliographs  are  now  used  daily  in  India,  Sicily,  Spain, 
Germany,  England,  and  the  United  States. 


CHAPTER  XII 
THE  YERKES  OBSERVATORY 

THE  formulation  of  the  theory  of  natural  selection  by 
Darwin  was  the  result  of  an  extensive  series  of  closely  cor- 
related investigations,  covering  a  broad  field.  His  object 
was  not  merely  to  bring  together  a  great  collection  of  plants 
or  animals,  describe  their  peculiarities,  and  confer  upon  them 
appropriate  names.  To  Darwin  each  of  these  plants  and 
animals  might  be  of  great  interest.  But  brilliant  plumage, 
unusual  form,  and  other  distinctive  peculiarities  were  of 
importance  to  him  mainly  because  of  their  bearing  upon  the 
question  of  development,  or  the  possible  relationship  of  the 
particular  specimen  to  others.  It  is  obvious  that  a  study  of 
such  relationships  must  greatly  enhance,  rather  than  dimin- 
ish, the  interest  of  the  investigator  in  the  peculiarities  which 
distinguish  species.  Having  in  mind  a  governing  principle, 
he  may  detect,  through  the  aid  of  delicate  markings  or  minute 
modifications  of  form  which  might  otherwise  be  inappre- 
ciable, the  evidences  of  development  which  constitute  the 
prime  object  of  his  search. 

Similar  tendencies  toward  unification  and  correlation  have 
shown  themselves  in  every  department  of  science.  Co-opera- 
tive undertakings  on  a  large  scale,  which  have  enlisted  the 
best  efforts  of  scientific  men  in  all  parts  of  the  world,  are 
common  at  the  present  time.  It  may  confidently  be  pre- 
dicted that  the  future  will  see  such  work  greatly  extended, 
and  that  the  various  agencies  which  can  thus  be  employed 
to  advance  science  will  be  utilized  in  an  increasingly  effec- 
tive manner. 

In  astronomical  and  astrophysical  research  the  opportu- 

97 


STELLAR  EVOLUTION 


nities  for  co-operation  and  correlation  are  unusually  good, 
and  have  yielded  many  important  results.  The  impossibility 
of  completing  at  any  one  observatory  the  extensive  investi- 
gations required  for  the  solution  of  large  cosmical  problems, 
and  the  advantages  which  may  result  from  the  discussion  of 
observations  made  simultaneously  or  at  stated  intervals  from 
stations  differing  widely  in  geographic  position,  altitude,  or 
climatic  conditions,  render  co-operation  essential  in  many 
cases.  Plans  for  international  co-operation  in  solar  research 
are  mentioned  elsewhere.  An  attempt  to  provide  for  the 
closest  possible  correlation  of  work  within  a  single  observa- 
tory is  also  described  in  this  book. 

In  establishing  an  observatory,  either  one  of  two  policies, 
both  represented  in  existing  institutions,  may  be  adopted. 
On  the  one  hand,  attention  may  be  directed  to  the  prosecu- 
tion of  individual  researches  or  extensive  routine  investiga- 
tions, not  necessarily  closely  related  to  one  another,  but  each 
constituting  an  important  contribution  to  knowledge.  On 
the  other  hand,  a  single  large  problem  may  be  chosen,  and 
all  individual  investigations  planned  so  as  to  lead  as  directly 
as  possible  toward  its  solution.  The  observations  required 
may  be  very  diverse,  and  cover  a  broad  field.  Each,  how- 
ever, to  be  most  effective  for  its  purpose,  must  be  chosen  with 
special  reference  to  the  existing  needs,  and  the  general  pro- 
gramme must  be  revised  from  time  to  time,  in  the  light  of 
every  important  advance. 

The  Yerkes  Observatory  may  serve  as  an  example  of  an 
institution  in  which  extensive  individual  investigations, 
differing  widely  in  character,  comprise  the  programme  of 
research.1  Its  scheme  of  work  was  based  on  a  deliberate 
intention  to  realize  the  fullest  possible  advantages  of  the 
40-inch  refractor  in  the  diverse  researches  for  which  it  is 

iln  the  astrophysical  work,  however,  an  effort  was  made  to  correlate  the  solar, 
stellar,  and  laboratory  investigations. 


THE  YERKES  OBSERVATORY  99 

peculiarly  adapted.  The  object  of  the  Mount  Wilson  Solar 
Observatory  of  the  Carnegie  Institution,  however,  is  to  con- 
centrate its  entire  attention  upon  the  study  of  the  Sun  and 
the  problem  of  stellar  evolution. 

After  the  spectroheliograph  had  been  tested  at  the  Ken- 
wood Observatory,  it  seemed  certain  that  this  method  was 
capable  of  further  extension,  and  the  desirability  of  securing 
better  instrumental  facilities  accordingly  presented  itself. 
The  establishment  of  the  new  University  of  Chicago  appeared 
to  offer  the  best  prospects  in  this  direction.  The  opportunity 
of  purchasing  two  disks  of  glass  for  the  objective  of  a 
40-inch  refractor  was  encountered  in  1893.  This  glass  had 
been  ordered  three  years  before  for  a  telescope  to  be  erected 
on  Mount  Wilson  in  southern  California — an  odd  coinci- 
dence in  the  light  of  subsequent  events.  As  funds  were  not 
available  for  the  completion  of  the  California  project  the 
glass  disks,  then  in  the  hands  of  Alvan  Clark  &  Sons,  were 
obtainable.  The  opportunity  was  an  unusual  one,  since  the 
disks  were  of  the  largest  size  and  of  the  most  perfect  optical 
glass.  After  several  unsuccessful  attempts  to  secure  the 
funds  from  other  sources,  the  matter  was  placed  before  Mr. 
Charles  T.  Yerkes  by  President  Harper.  He  promptly  sig- 
nified his  desire  to  provide  for  the  construction  of  a  40-inch 
refractor.  The  glass  was  purchased,  a  contract  arranged 
with  Clark  to  complete  the  object-glass,  and  the  mounting 
ordered  from  Warner  &  Swasey.  The  construction  of  the 
Yerkes  Observatory  was  undertaken  in  1895  and  completed 
in  1897. 

The  gift  which  provided  for  the  Yerkes  Observatory  was 
made  before  the  University  of  Chicago  had  opened  its  doors 
to  students.  In  fact,  the  original  idea  of  establishing  a  col- 
lege, rather  than  a  university,  had  hardly  been  outgrown, 
and  the  question  of  the  recognition  to  be  accorded  to  research 
was  still  a  cause  of  concern  to  the  members  of  the  rapidly 


100  STELLAR  EVOLUTION 

enlarging  faculty.  A  narrow  view  of  the  future  on  the  part 
of  the  trustees  might  have  led  to  the  erection  of  the  obser- 
vatory in  Chicago,  and  its  use  for  the  purposes  of  instruction 
rather  than  for  those  of  research.  Fortunately,  a  different 
policy  prevailed.  It  was  recognized  that  the  40-inch  tele- 
scope should  be  exclusively  devoted  to  investigation,  and  that 
a  site  in  the  immediate  neighborhood  of  the  university 
grounds  would  prevent  its  effective  use.  It  was  accord- 
ingly decided  to  secure  a  site  in  the  most  favorable  location 
within  a  reasonable  distance  of  Chicago,  and  a  tract  of 
land  in  Wisconsin,  on  the  shore  of  Lake  Geneva,  was  finally 
selected. 

The  plan  of  the  building  shows  the  influence  of  the  Lick 
Observatory  and  the  Astrophysical  Observatory  of  Potsdam, 
both  of  which  embody  many  admirable  features.  The  adopted 
form  of  a  Roman  cross  permitted  the  three  domes  to  be  sepa- 
rated to  such  an  extent  that  they  practically  do  not  interfere 
in  the  least  with  one  another  (Plate  XLIV).  The  desire  of 
the  donor  for  an  ornate  structure,  and  the  decision  of  the 
architect  to  introduce  rather  florid  embellishments  of  terra- 
cotta, led  to  the  use  of  brick  as  a  building  material.  This  was 
quite  in  accordance  with  convention,  but  in  conflict  with  the 
condition,  well  known  to  astronomers,  that  the  temperature 
within  an  observing-room  should  be  as  nearly  as  possible  the 
same  as  the  temperature  of  the  outer  air.  The  massive  brick 
wall  of  the  great  tower  in  which  the  40-inch  telescope  is 
mounted  is  therefore  decidedly  inferior  to  a  light  steel  con- 
struction, with  a  thin  metallic  wall,  shielded  from  the  Sun  by 
an  outer  wall  of  similar  type.  Architectural  considerations, 
however,  have  weighed  as  heavily  in  nearly  all  of  the  world's 
largest  observatories,  and  the  complete  freedom  of  action, 
subsequently  experienced  at  Mount  "Wilson,  had  not  yet  .been 
attained. 

The  engineering  problems  presented  by  the  great  size 


THE  YERKES  OBSERVATORY  '  101 

of  the  Yerkes  telescope,  and  of  the  dome  under  which  it  was 
mounted,  were  such  as  to  tax  the  efforts  of  even  so  skilful  a 
firm  as  that  of  Warner  &  Swasey,  to  whom  the  work  was 
intrusted.  The  admirable  qualities  of  the  mounting  of  the 
Yerkes  telescope  show  the  advantage  of  the  experience  gained 
by  them  in  constructing  the  Lick  telescope.  The  dome  and 
rising-floor,  after  several  faults  of  design  and  construction 
had  been  remedied,  also  performed  very  well.  Thoroughly 
tested  by  continuous  use,  by  night  and  by  day,  for  a  period  of 
ten  years,  the  entire  plant  may  certainly  be  considered  to 
reflect  much  credit  upon  these  well-known  engineers. 

The  40-inch  telescope,  and  other  instruments  of  the 
Yerkes  Observatory,  have  already  been  described  in  previous 
chapters,  but  a  few  additional  details  may  be  of  interest. 
The  object-glass,  which  was  put  in  place  only  a  few  weeks 
before  the  death  of  Alvan  Gr.  Clark,  the  last  member  of  the 
celebrated  firm  of  Alvan  Clark  &  Sons,  is  made  up  of  two 
lenses.  The  outer  lens,  made  of  crown  glass,  is  double  con- 
vex in  form  (Plate  XLV).  The  inner  lens,  separated  from 
the  other  by  a  distance  of  about  eight  inches,  is  plano-con- 
cave, and  made  of  flint  glass.  The  total  weight  of  the  glass 
in  the  two  lenses  is  about  500  pounds.  The  rough  glass 
disks,  from  which  the  lenses  were  fashioned  by  the  Olarks, 
were  made  by  Mantois,  of  Paris.  The  glass  is  of  extra- 
ordinary purity  and  transparency,  but  in  spite  of  this  fact  it 
absorbs  much  light,  on  account  of  its  considerable  thickness 
(about  three  inches  in  all).  The  conditions  are  very  dif- 
ferent from  those  of  a  reflecting  telescope,  where  much  less 
perfect  glass  is  required,  since  in  the  latter  case  the  light  is 
reflected  from  a  layer  of  pure  silver  on  the  front  surface  and 
therefore  suffers  no  absorption  in  transmission  (though  some 
light  is  lost  in  reflection).  It  has  already  been  pointed 
out  that  refracting  and  reflecting  telescopes  have  their  own 
peculiar  advantages  and  defects.  The  choice  of  the  one  or 


102  STELLAR  EVOLUTION 

the  other  must  depend  upon  the  needs  of  the  work  for  which 
it  is  required. 

In  order  to  direct  the  40-inch  telescope  to  a  faint  star, 
the  sidereal  time,  as  well  as  the  right  ascension  and  declina- 
tion of  the  star,  must  be  known.  After  the  opening  in  the 
dome  has  been  turned  toward  the  proper  quarter  of  the 
heavens,  the  telescope  is  moved  in  right  ascension  (i.  e., 
around  the  polar  axis,  which  is  parallel  to  the  Earth's  axis) 
until  the  hour  circle,  attached  to  this  axis,  indicates  the 
proper  reading.  This  reading  is  determined  by  taking  the 
difference  between  the  sidereal  time  and  the  right  ascension 
of  the  star.  The  result  gives  the  distance  of  the  star  from 
the  meridian,  expressed  in  hours  and  minutes  of  time.  The 
motion  of  the  telescope  in  right  ascension  is  produced  by 
means  of  an  electric  motor,  controlled  by  a  rope  running  down 
the  north  face  of  the  iron  column  and  easily  reached  from 
the  rising-floor.  The  next  operation  is  to  move  the  telescope 
in  declination  (i.  e.,  around  an  axis  at  right  angles  to  the 
polar  axis)  until  the  declination  circle  indicates  the  proper 
reading,  so  many  degrees  north  or  south  of  the  equator.  If 
the  eye-end  of  the  telescope  is  then  too  high  to  be  reached 
by  the  observer  on  the  rising-floor,  the  floor  is  raised  by 
means  of  an  electric  motor,  controlled  by  a  switch  near  the 
telescope  column.  An  adjoining  switch  controls  the  motor 
which  turns  the  dome.  On  looking  into  the  eye-piece  the 
star  will  be  found  in  the  field,  provided  the  setting  has  been 
accurately  made.  The  telescope  is  next  clamped  in  right 
ascension  and  declination.  It  will  then  be  carried  by  the 
driving-clock,  which  causes  the  polar  axis  to  rotate  through 
a  complete  revolution  in  twenty-four  hours.  The  apparent 
motion  of  the  star  in  the  heavens  is  thus  counteracted,  and 
the  image  remains  fixed  in  the  field  of  view,  where  it  may 
be  studied  in  any  way  desired. 

If,  for  example,  the  observer  wishes  to  measure  the  posi- 


THE  YERKES  OBSERVATORY  103 

tion  of  the  star  with  respect  to  other  stars  in  its  neighbor- 
hood, this  is  accomplished  by  means  of  a  position  micrometer, 
in  which  a  fine  spider  line  can  be  moved  through  the  neces- 
sary distance  by  a  micrometer  screw.  The  value  of  one  divi- 
sion of  the  micrometer  head,  in  seconds  of  arc,  is  previously 
determined  by  measuring  the  distance  between  two  known 
stars,  whose  positions  have  been  accurately  fixed  by  means 
of  a  meridian  circle.  Burnham's  admirable  observations 
of  double  stars  with  the  40-inch  telescope  have  all  involved 
the  accurate  micrometric  measurement  of  the  distance  sepa- 
rating the  stars  of  each  pair.  The  position  angle  of  the  line 
joining  the  two  stars,  with  reference  to  a  north-and-south 
line  in  the  heavens,  is  also  measured  in  each  case  with  the 
aid  of  a  divided  circle  attached  to  the  micrometer.  On 
account  of  the  large  aperture  of  the  telescope,  it  is  possible 
to  separate  with  it  stars  about  one-tenth  of  a  second  of  arc 
apart,  provided  the  atmospheric  conditions  are  sufficiently 
good  for  the  purpose.  As  the  distance  between  the  two 
images  in  the  principal  focus  of  the  telescope  would,  in  this 
case,  amount  to  but  little  over  one  three-thousandth  part  of 
an  inch,  it  is  obvious  that  the  best  of  conditions  are  required 
for  such  exacting  work. 

Barnard's  observations  with  the  Yerkes  telescope  have  also 
involved  the  constant  use  of  the  micrometer.  The  difficulty  of 
the  work,  and  the  patience  required  to  pursue  it,  can  be  ima- 
gined when  it  is  remembered  that  Barnard  has  measured  the 
positions  of  hundreds  of  stars  in  such  a  closely  crowded  cluster 
as  that  illustrated  in  Plate  XIX.  In  such  work  as  this  the 
observer  remains  standing  throughout  the  entire  night.  It 
should  also  be  remembered  that  in  the  open  dome  the  tem- 
perature sometimes  falls  to  — 20°  F.  in  the  rigorous  Wiscon- 
sin winters.  It  is  evident  that  only  the  greatest  interest  and 
devotion  on  the  part  of  the  observer  can  permit  him  to  make 
accurate  measures,  night  after  night,  under  such  conditions. 


104  STELLAR  EVOLUTION 

We  have  already  seen  (in  chap,  xi)  how  the  Rumford 
spectroheliograph  is  used  with  the  Yerkes  telescope.  As  the 
spectroheliograph  weighs  about  700  pounds,  and  must  be 
attached  each  morning  and  taken  off  at  night,  special  arrange- 
ments are  required  to  facilitate  this  work.  Each  heavy 
instrument  used  in  conjunction  with  the  telescope  is  mounted 
on  a  carriage,  which  stands  on  the  rising-floor.  When  the 
change  is  to  be  made  from  one  attachment  to  another,  the 
floor  is  raised  to  its  highest  position  and  the  telescope  tube 
firmly  anchored  to  it  by  means  of  a  steel  bar.  This  is  to 
obviate  any  danger  of  accident  when  the  balance  of  the  tube 
is  temporarily  disturbed.  The  carriage  bearing  the  spectro- 
heliograph is  brought  to  the  eye-end  of  the  telescope,  the 
spectroheliograph  clamped  to  its  supporting  ring,  and  over 
700  pounds  of  iron  weights  removed  from  the  telescope 
tube.  This  restores  the  balance,  which  must  be  adjusted 
to  a  nicety. 

The  Bruce  spectrograph  (Plates  XL VI  and  LXXVIII) 
is  used  by  Frost  for  the  photographic  study  of  stellar  spectra. 
The  image  of  a  star  is  formed  on  the  slit  of  the  spectrograph, 
which  is  about  one-thousandth  of  an  inch  in  width.  The 
light  then  passes  to  a  colliinator  lens,  which  renders  the  rays 
parallel.  Three  large  prisms,  next  traversed  by  the  rays, 
bend  them  through  an  angle  of  180°  and  disperse  them 
into  a  spectrum.  The  camera  lens  forms  an  image  of  the 
spectrum  upon  the  photographic  plate.  Throughout  the 
exposure,  which  may  be  continued  several  hours,  the  ob- 
server watches  the  star  image  and  keeps  it  accurately  on 
the  slit,  any  imperfections  in  the  driving  of  the  telescope 
being  corrected  by  means  of  electric  slow  motions.  In  order 
to  eliminate  the  effect  of  the  changing  temperature  in  the 
open  dome,  the  spectrograph  is  inclosed  in  a  tight-fitting 
case,  the  interior  of  which  is  maintained  at  a  uniform  tem- 
perature by  electric-heating  coils. 


THE  YERKES  OBSERVATORY  105 

In  order  to  determine  the  position  of  the  lines  in  a 
spectrum,  a  suitable  comparison  spectrum  is  required.  This 
is  obtained  by  passing  an  electric  spark  between  poles  of 
titanium  or  iron  and  photographing  the  spectrum  of  the 
spark  on  each  side  of  that  of  the  star.  An  enlargement  of 
one  of  Frost  and  Adams'  photographs  of  ?;  Leonis,  made  in 
this  way,  is  reproduced  in  Plate  XLVII.  It  will  be  seen  that 
the  lines  of  the  comparison  spectrum  are  shifted  a  slight 
distance  toward  the  red  (right),  with  reference  to  the  corre- 
sponding lines  in  the  star.  This  shift  is  due  to  the  motion 
of  the  star  away  from  the  Earth,  which  in  this  instance 
amounts  to  28  kilometers  per  second.  On  account  of  its 
orbital  motion,  the  Earth  was  moving  toward  the  star  on 
this  date  at  the  rate  of  26  kilometers  per  second.  Hence  the 
velocity  of  rj  Leonis  with  respect  to  the  Sun  was  +  2  kilo- 
meters per  second. 

Such  displacements  of  the  lines  provide  the  only  means 
of  determining  whether  a  star  is  approaching  or  receding 
from  the  Earth.  This  method,  first  tried  visually  by  Hug- 
gins,  was  successfully  adopted  for  photographic  work  by 
Vogel,  and  subsequently  greatly  refined  by  Campbell,  who 
applied  it  with  remarkable  success  at  the  Lick  Observatory. 
In  the  hands  of  Campbell,  Frost,  and  others,  it  has  resulted  in 
the  discovery  of  many  " spectroscopic  binaries" — double  stars 
in  which  the  component  members  are  revolving  at  such  great 
velocities  that  they  periodically  displace  the  lines  in  their 
spectra.  In  most  of  these  binaries  one  of  the  components 
is  a  dark  star.  Our  only  clue  to  their  duplicity  is  thus  fur- 
nished by  the  fact  that  the  lines  move  back  and  forth  with 
respect  to  the  comparison  lines,  the  displacement  being 
toward  the  violet  when  the  star  is  approaching,  and  toward 
the  red  when  it  is  receding  from  the  Earth.  In  a  subsequent 
chapter  it  will  appear  how  photographs  of  stellar  spectra 
are  used  in  the  study  of  stellar  evolution. 


106  STELLAR  EVOLUTION 

The  Rumford  spectroheliograph  and  the  Bruce  spectro- 
graph  were  constructed  in  the  instrument  shop  of  the  Yerkes 
Observatory.  It  had  long  been  customary  for  observatories 
to  provide  means  of  repairing  their  own  instruments,  but  the 
work  of  construction  had,  as  a  rule,  been  left  to  the  profes- 
sional instrument-makers.  At  the  Yerkes  Observatory  a  well- 
equipped  shop  was  not  only  a  convenience,  but  a  necessity. 
The  funds  given  for  the  establishment  of  the  observatory  did 
not  provide  for  a  general  equipment  of  minor  instruments. 
In  the  absence  of  the  means  of  purchasing  instruments,  the 
only  alternative  was  to  construct  them.  Fortunately,  a 
number  of  machine  tools  had  formed  part  of  the  equipment 
of  the  Kenwood  Observatory  and  were  immediately  available. 
The  appropriations  of  the  University  of  Chicago  permitted  a 
skilled  instrument-maker  to  be  regularly  employed,  and  spe- 
cial gifts,  received  from  various  sources  in  subsequent  years, 
sometimes  enabled  us  to  keep  several  men  at  work.  The 
instrument  shop,  at  first  under  the  direction  of  Wadsworth 
and  subsequently  under  Ritchey  (who  was  in  charge  of  the 
optical  shop  from  the  beginning),  proved  to  be  indispensable 
to  the  success  of  the  Observatory's  work.  Not  only  the  instru- 
ments already  mentioned,  but  also  the  2-foot  reflector,  the 
Snow  telescope,  a  3j-inch  transit  instrument,  spectroscopic 
and  other  apparatus  used  in  the  laboratory,  and  many  special 
instruments  and  appliances  employed  with  the  40-inch  tele- 
scope and  in  other  departments  of  the  work,  came  from  this 
source.  It  may  be  said  that  in  a  large  astrophysical  observa- 
tory, where  new  types  of  instruments  are  constantly  being 
devised,  a  well-equipped  instrument  shop  is  essential  if  the 
best  results  are  to  be  obtained.  This  is  largely  because  of 
the  advantage  of  having  the  instruments  constructed  under 
the  immediate  supervision  of  the  men  who  are  responsible  for 
their  design. 

The   optical   shop    was   another    feature   of  the  Yerkes 


THE  YERKES  OBSEKVATOEY  107 

Observatory  which  contributed  in  a  most  important  manner 
to  its  work.  Here  Bitchey  made  numerous  mirrors — plane, 
concave,  and  convex — for  use  in  the  Snow  telescope,  the 
2-foot  reflector,  and  other  instruments,  and  here  also  he  did 
a  large  part  of  the  work  on  the  60-inch  mirror,  which  was 
subsequently  transferred  to  the  Solar  Observatory.  As  the 
methods  employed  in  grinding  and  polishing  this  mirror  are 
described  in  chap,  xxiii,  no  further  mention  will  be  made  of 
them  here.  It  may  be  said,  however,  that  many  special 
investigations  set  on  foot  at  the  Yerkes  Observatory  could 
not  have  been  undertaken  without  the  unique  advantages 
afforded  by  the  optical  shop. 

Still  another  feature  of  the  Yerkes  Observatory,  which 
was  subsequently  repeated,  in  improved  form,  at  Mount 
Wilson,  is  the  spectroscopic  laboratory,  in  which  various 
solar  and  stellar  phenomena  are  imitated  experimentally. 
Apparatus  for  producing  sparks  between  metallic  poles  in 
air,  in  liquids,  and  in  compressed  gases  is  arranged  on  the 
circumference  of  a  circular  table.  Low-voltage  arcs  are  also 
provided,  the  purpose  of  the  equipment  being  to  furnish 
means  of  varying,  between  wide  limits,  the  conditions  of 
temperature  and  pressure,  and  of  gaseous  or  liquid  environ- 
ment, in  which  the  metallic  vapors  emit  their  characteristic 
radiations.  By  setting  at  the  proper  angle  a  plane  mirror, 
mounted  at  the  center  of  the  table,  light  from  any  source 
can  be  reflected  to  a  concave  mirror,  which  forms  an  image  of 
the  source  on  the  slit  of  a  large  concave  grating  spectrograph. 
The  most  extensive  single  investigation  made  in  this  labora- 
tory was  a  study  of  the  spectrum  of  the  spark  in  liquids  and 
compressed  gases,  to  test  Wilsing's  pressure  theory  of 
temporary  stars. 

In  the  diversified  work  of  the  Yerkes  Observatory  the 
desire  to  attack  the  problem  of  stellar  evolution  in  the  most 
effective  manner  was  not  forgotten.  Experience  with  the 


108  STELLAR  EVOLUTION 

large  concave  grating  of  the  Kenwood  Observatory  had 
furnished  convincing  evidence  of  the  advantages  of  fixed 
instruments  mounted  on  piers,  and  the  beautiful  resolution  of 
the  solar  spectrum  with  this  apparatus  made  observations  of 
stellar  spectra  with  small  prism  spectroscopes  seem  unsatis- 
factory. It  was  felt  from  the  first  that  every  effort  should  be 
made  to  devise  a  telescope  capable  of  bringing  a  large  and 
well-defined  solar  image,  or  a  sharp  and  brilliant  stellar 
image,  into  a  laboratory,  where  it  could  be  observed  to  the 
best  possible  advantage,  with  appliances  too  large  or  too 
heavy  for  use  with  moving  telescopes.  It  seemed  clear  that, 
if  this  desire  could  be  realized,  and  if  the  full  advantages 
of  reflecting  telescopes  for  astrophysical  research  could  be 
attained,  the  means  thus  provided  should  render  possible  a 
well-directed  attack  on  the  problem  in  mind. 

The  work  of  the  Rumford  spectroheliograph  showed  that 
the  further  development  of  this  instrument  must  involve  a 
considerable  increase  in  dispersion,  so  as  to  permit  the  use 
of  the  narrower  dark  lines.  This  meant  an  instrument  of 
large  dimensions,  necessarily  to  be  mounted  in  a  fixed  posi- 
tion, since  it  could  not  be  attached  to  a  moving  telescope 
tube.  Another  piece  of  work  pointed  to  the  same  require- 
ment. At  the  Kenwood  Observatory  attempts  were  made  to 
photograph  the  spectra  of  sun-spots,  and  negatives  were 
secured  showing  a  few  of  the  more  conspicuous  widened  lines. 
The  need  of  a  larger  solar  image  for  this  work  was  met  by 
the  Yerkes  telescope.  A  marked  improvement  in  the  photo- 
graphs resulted.  However,  it  was  clear  that  photographs  of 
spot  spectra  suitable  for  the  most  refined  investigations  could 
not  be  obtained  without  the  use  of  a  spectrograph  of  much 
higher  dispersion.  For  satisfactory  results  a  spectrograph 
of  at  least  10  feet  focal  length  was  needed,  and  this  could 
not  be  attached  to  the  moving  telescope  tube.  Here,  again, 
was  another  argument  for  the  fixed  type  of  telescope. 


THE  YERKES  OBSERVATORY  109 


The  work  of  constructing  such  an  instrument  was  accord- 
ingly taken  up.  The  original  purpose  of  building  a  heliostat 
was  modified,  through  the  recognition  of  the  superior  advan- 
tages of  the  coelostat,  introduced  by  Turner  for  eclipse 
observations.  A  30-inch  coelostat,  designed  by  Bitchey, 
was  constructed  in  the  instrument-  shop  of  the  Yerkes 
Observatory.  This  was  destroyed  by  fire,  but  a  gift  from 
Miss  Snow  of  Chicago,  in  memory  of  her  father,  provided 
the  funds  required  for  the  Snow  telescope.  In  the  prelimi- 
nary tests  of  this  instrument  at  the  Yerkes  Observatory  the 
images  were  not  very  satisfactory,  but  it  subsequently  gave 
admirable  results  at  Mount  Wilson. 

In  establishing  the  Carnegie  Institution  at  Washington, 
Mr.  Carnegie  gave  expression  to  his  appreciation  of  the  fact 
that  some  of  the  most  fundamental  needs  of  scientific  research 
could  not  be  supplied  by  existing  agencies.  As  a  rule,  a 
university  must  build  its  observatory  or  biological  laboratory 
near  at  hand,  rather  than  at  a  site  chosen  because  of  atmos- 
pheric advantages  or  the  richness  of  the  local  fauna  and  flora. 
Its  funds,  usually  given  for  specific  purposes,  are  likely  to  be 
unavailable,  or  perhaps  inadequate,  to  provide  a  sufficiently 
large  corps  of  investigators,  devoted  to  research.  If,  through 
the  efforts  of  one  of  its  faculty,  a  new  and  promising  instru- 
ment is  projected,  the  trustees  may  not  be  in  a  position  to 
supply  the  financial  means  required  to  construct  it.  Such 
conditions  result  from  the  very  nature  of  a  university's 
work,  and  consequently  affect,  in  some  degree,  the  policy 
of  even  so  progressive  an  institution  as  the  University  of 
Chicago,  where  the  authorities  strongly  favor  original  in- 
vestigation. The  Carnegie  Institution,  devoted  exclusively 
to  the  furtherance  of  research,  is  not  thus  hampered.  It 
therefore  came  about  that  this  new  Institution,  with  the 
cordial  co-operation  of  the  University  of  Chicago,  made  pro- 
vision for  the  continuation  and  development  of  the  work  set 


110  STELLAR  EVOLUTION 

on  foot  at  the  Kenwood  and  Yerkes  Observatories.  A  com- 
mittee, appointed  to  report  on  the  advisability  of  establishing 
an  observatory  for  solar  research,  and  another  observatory  for 
observations  of  the  southern  heavens,  favored  both  of  these 
projects.  A  careful  test  of  various  sites  in  the  United  States 
and  in  Australia,  made- at  the  request  of  the  committee  by 
Hussey,  led  to  the  provisional  selection  of  Mount  Wilson 
(5,886  feet),  near  Pasadena  in  southern  California,  as  the 
site  for  the  proposed  solar  observatory.  An  appropriation, 
granted  by  the  Carnegie  Institution  in  1904,  furnished  the 
means  of  sending  an  expedition  from  the  Yerkes  Observatory 
to  Mount  Wilson.  The  Snow  telescope  was  erected  on  the 
mountain,  in  a  new  type  of  house  especially  designed  for  it. 
An  instrument  shop  was  established  in  Pasadena  for  the 
construction  of  the  spectroheliographs  and  other  apparatus 
required  for  use  with  the  Snow  telescope.  In  December, 
1904,  the  Carnegie  Institution  decided  to  establish  a  solar 
observatory  of  its  own  on  Mount  Wilson.  Through  the 
courtesy  of  the  authorities  of  the  Yerkes  Observatory  and 
the  University  of  Chicago,  the  Snow  telescope  was  retained 
on  the  mountain,  and  has  since  been  purchased  by  the  Solar 
Observatory.  The  optical  work  on  the  60-inch  mirror,  which 
was  also  acquired  by  the  Solar  Observatory,  was  resumed 
by  Ritchey  in  the  optical  shop  at  Pasadena.  He  also 
designed  the  mounting  for  this  telescope,  and  the  work  of 
constructing  it  was  soon  undertaken. 


CHAPTER  XIII 
ASTRONOMICAL  ADVANTAGES  OP  HIGH  ALTITUDES 

THE  recognition  of  the  advantages  of  making  astronomical 
observations  at  high  altitudes  goes  back  to  the  time  of  New- 
ton, who  wrote  as  follows  in  his  Opticks  (third  edition,  p.  98) : 

If  the  Theory  of  making  Telescopes  could  at  length  be  fully 
brought  into  practice,  yet  there  would  be  certain  Bounds  beyond 
which  Telescopes  could  not  perform.  For  the  Air  through  which 
we  look  upon  the  Stars,  is  in  a  perpetual  Tremor;  as  may  be  seen 
by  the  tremulous  Motion  of  Shadows  cast  from  high  Towers,  and  by 
the  twinkling  of  the  fix'd  stars.  *  *  *  The  only  remedy  is  a  most 
serene  and  quiet  Air,  such  as  may  perhaps  be  found  on  the  tops  of 
the  highest  Mountains  above  the  grosser  Clouds. 

It  will  be  observed  from  these  remarks  that  a  clear  and 
transparent  sky  is  not  the  only  need  of  the  astronomer.  In 
their  passage  through  our  atmosphere  the  rays  which  are 
united  by  a  telescope  to  form  the  image  of  a  star  traverse 
different  paths,  depending  upon  their  color.  For  air,  like 
water  or  glass,  though  in  a  less  degree,  is  a  refracting  medi- 
um; i.  e.,  a  ray  of  light  entering  it  is  bent  from  its  straight 
course,  and  the  amount  of  its  bending  depends  upon  the 
color  of  the  ray,  just  as  in  the  case  of  a  prism.  Violet  light 
suffers  the  greatest  refraction,  and  red  light  the  least.  Ob- 
viously, then,  rays  of  different  colors  coming  to  a  telescope 
from  a  star  do  not  pursue  the  same  path.  Since  the  degree 
of  refraction  depends  upon  the  temperature  of  the  air,  and 
since,  under  ordinary  conditions,  the  temperature  is  chan- 
ging in  an  irregular  manner,  we  thus  see  why  a  star  twinkles 
and  undergoes  rapid  change  of  color.  For  the  red  rays  may 
be  momentarily  reduced  in  brightness,  through  a  change  in 

111 


112  STELLAK  EVOLUTION 

refraction  of  the  air  through  which  they  pass.  The  star 
would  thus  appear  blue  for  the  time  being.  The  next 
instant  the  intensity  of  the  blue  light  might  be  reduced, 
causing  the  star  to  seem  red.  Since  the  length  of  the  light- 
path  and  the  degree  of  refraction  increase  toward  the  hori- 
zon, the  twinkling  of  stars,  which  frequently  disappears  alto- 
gether at  the  zenith,  is  most  apparent  at  low  altitudes. 

As  the  effect  of  twinkling  is  so  apparent  to  the  eye,  it  is 
easy  to  see  that  it  may  be  greatly  magnified  in  a  telescope 
and  produce  serious  interference  with  observations.  The 
star  image,  instead  of  being  a  minute,  sharply  denned  point, 
usually  appears  in  the  telescope  enlarged,  confused,  and 
tremulous.  The  component  members  of  close  double  stars, 
though  easily  within  the  resolving  power  of  the  telescope, 
under  such  conditions  may  overlap  and  appear  as  one.  Simi- 
larly the  minute  surface  details  of  the  Moon  or  planets  may 
be  entirely  obliterated  by  atmospheric  disturbance.  It  is  as 
though  the  astronomer  were  forced  to  observe  the  heavenly 
bodies  from  the  bottom  of  an  ocean,  not  calm  and  tranquil 
throughout  its  mass,  but  constantly  disturbed  by  currents  of 
various  directions  and  at  different  depths,  and  by  irregu- 
larities of  density  arising  from  unequal  temperatures. 

It  sometimes  happens  that  excellent  definition  of  tele- 
scopic images  is  obtained  through  smoke  or  haze,  under  cir- 
cumstances which  might  appear  to  be  wholly  unsuitable  for 
astronomical  work.  For  certain  kinds  of  observations,  where 
perfect  definition  is  all  important  and  brightness  of  the  image 
of  less  consequence,  the  lack  of  transparency  occasioned  by 
hazy  air  does  no  harm.  But  in  most  classes  of  work  particles 
suspended  in  the  atmosphere  not  only  reduce  the  inten- 
sity of  the  light,  but  produce  serious  interference  through 
scattering  of  the  rays.  The  brightness  of  the  sky  near  the 
Sun,  for  example,  increases  greatly  with  the  number  of  dust 
or  smoke  particles  in  the  air.  In  visual  observations  of  the 


ADVANTAGES  OF  HIGH  ALTITUDES  113 

details  of  sun-spots  this  might  not  be  harmful;  but  the  visi- 
bility of  the  prominences  is  seriously  reduced  when  they  are 
seen  against  a  brilliant  background  of  sky.  The  brightness 
of  stars  is  also  much  affected  by  haziness  of  the  atmosphere. 

Even  on  a  clear  and  transparent  night  the  stars  are  less 
brilliant  at  sea  level  than  when  seen  from  the  summit  of  a 
high  mountain.  For  the  air  itself  is  a  powerful  absorbing 
medium  and  reduces,  more  than  we  ordinarily  realize,  the 
brightness  of  objects  seen  through  it.  Illustrations  of  the 
relative  advantages  of  photographing  stars  at  altitudes  of 
1,200  and  6,000  feet  respectively  is  given  in  Plates  LVI 
and  LVIL 

The  difficulties  in  astronomical  observations  arising  from 
atmospheric  disturbances  increase  with  the  aperture  of  the 
telescope  employed.  This  is  because  the  rays  falling  on 
opposite  sides  of  a  large  object-glass  traverse  more  widely 
separated  paths  than  those  united  by  a  small  object-glass. 
They  are  thus  liable  to  greater  atmospheric  disturbance,  on 
account  of  the  difference  in  the  conditions  governing  the 
refraction  of  the  light  along  the  two  paths.  The  disturbances 
of  the  air  take  the  form  of  more  or  less  regular  waves.  With 
an  aperture  which  is  small  compared  with  the  length  of  one 
of  these  waves,  the  effect  on  the  image  might  not  be  great. 
If,  however,  several  waves  were  included  within  the  aperture, 
the  confusion  might  be  very  marked  indeed.  Hence  large 
telescopes  require  better  conditions  than  small  ones. 

In  selecting  the  site  of  the  Yerkes  Observatory,  practical 
considerations  necessarily  limited  the  choice.  It  was  essential 
that  the  observatory  should  be  situated  within  easy  reach  of 
the  university,  and  this  fact  rendered  it  impossible  to  consider 
seriously  the  favorable  mountain  regions  which  were  known 
to  exist  in  the  extreme  western  part  of  the  United  States. 
The  chosen  site  has  many  advantages  over  points  in  the 
immediate  neighborhood  of  Chicago.  The  absence  of  smoke 


114  STELLAE  EVOLUTION 

and  the  brilliant  illumination  of  the  sky  produced  in  large 
cities  by  electric  lights,  the  freedom  from  vibration  arising 
from  railways  and  the  heavy  traffic  of  a  large  city,  and  the 
facilities  for  quiet  study  afforded  by  the  tranquil  life  of  the 
country,  were  important  recommendations  of  the  Lake 
Geneva  site.  The  observational  work  of  the  Yerkes  Obser- 
vatory has  been  sufficient  in  amount  and  quality  to  show 
that  more  valuable  material  can  be  secured  under  such 
atmospheric  conditions  than  can  be  adequately  discussed 
without  a  far  larger  staff  of  computers  than  the  observatory 
has  ever  been  able  to  employ.  It  goes  without  saying,  how- 
ever, that  a  better  site  would  have  been  preferable. 

But  it  must  not  be  supposed,  from  what  has  been  said, 
that  all  mountain  peaks  would  make  good  observing  stations. 
It  is  true  that  by  ascending  into  the  upper  atmosphere  the 
astronomer  may  escape  the  strong  absorption  exercised  by 
the  dense  air  of  lower  levels.  As  one  goes  up,  the  stars 
become  brighter  and  brighter,  especially  near  the  horizon, 
since  the  decrease  in  length  of  path  is  much  greater  in  this 
region  than  near  the  zenith.  Blue  and  violet  light  suffer 
more  from  atmospheric  absorption  than  the  red,  yellow,  and 
green  rays.  For  this  reason,  the  advantages  of  high  eleva- 
tions, so  far  as  transparency  is  concerned,  are  more  apparent 
in  photographic  than  in  visual  observations,  since  the  blue 
and  violet  rays  are  principally  concerned  in  the  production 
of  the  photographic  image. 

Thus,  from  the  standpoint  of  atmospheric  transparency, 
mountain  sites  may  always  be  considered  to  possess  advan- 
tages for  astronomical  work.  But  transparency  is  almost 
invariably  a  much  less  important  consideration  than  sharp- 
ness of  definition,  which  does  not,  by  any  means,  depend 
merely  upon  altitude.  In  the  first  place,  the  geographic 
location  of  the  mountain  in  question  is  a  most  important 
factor.  Long  periods  of  continuous  clear  weather,  enjoyed 


ADVANTAGES  OF  HIGH  ALTITUDES  115 

in  certain  favored  regions,  are  accompanied  by  a  uniformity 
of  atmospheric  conditions  unknown  in  countries  where  storms 
usually  prevail.  It  is  not  merely  that  clouds  and  rain  are 
less  common;  for,  if  this  were  the  only  important  considera- 
tion, a  clear  night  in  one  part  of  the  world  might  be  as  good 
for  astronomical  purposes  as  an  equally  clear  night  in  an- 
other. In  a  region  of  storms  the  disturbances  follow  one 
another  so  rapidly  that  during  the  intervening  periods  of 
clear  weather  the  atmosphere  rarely  has  time  to  settle  down 
to  a  calm,  homogeneous  state.  In  southern  California,  for 
example,  the  sky  is  almost  constantly  clear  for  many  months 
in  the  year,  and  the  uniformity  of  the  atmosphere  is  shown 
by  the  steadiness  of  the  barometer  and  the  low  average  wind 
velocity.  During  the  rainy  season,  however,  when  storms 
may  recur  in  rapid  succession,  the  atmosphere  in  such  a 
region  is  disturbed,  and  the  conditions  for  astronomical  work 
on  the  beautifully  transparent  nights  that  intervene  between 
storms  are  frequently  no  better  than  in  the  eastern  part  of 
the  United  States. 

Pike's  Peak  (14.147  feet)  affords  an  example  of  a  moun- 
tain site  poorly  adapted  for  astronomical  purposes.  In  June 
and  July  of  1893  I  spent  two  weeks  there,  in  company 
with  Keeler,  engaged  in  an  attempt  to  photograph  the  solar 
corona  without  an  eclipse.  Under  normal  conditions  the 
sky,  as  seen  from  the  peak,  is  of  a  deep  blue  by  day,  and 
very  transparent  by  night.  The  conditions,  therefore,  are 
favorable  for  work  in  which  transparency  is  the  only  important 
consideration.  Thus  Pike's  Peak  might  serve  very  .well  for 
the  measurement  of  the  solar  radiation,  were  it  not  for  the 
fact  that  during  the  summer  months  (always  the  most 
important  season  for  solar  work),  the  mountain  is  frequently, 
capped  by  clouds  through  a  considerable  part  of  the  day. 
On  many  of  the  nights  during  our  stay  the  sky  was  perfectly 
clear,  and  remained  so  until  about  nine  o'clock  in  the  morn- 


116  STELLAK  EVOLUTION 

ing.  Then  small  cumulus  clouds  would  begin  to  form  imme- 
diately around  the  peak,  and  by  noon  a  thunderstorm  would 
be  raging,  frequently  accompanied  by  a  light  fall  of  snow. 
In  these  storms  the  wind  rose  to  a  tremendous  velocity,  some- 
times as  great  as  seventy  miles  an  hour,  and  the  electrical 
phenomena  were  very  remarkable.  The  frequency  with  which 
these  storms  cut  off  all  solar  observations,  except  in  the  early 
morning,  illustrates  the  fact  that  even  for  work  on  the  solar 
radiation,  which  requires  a  clear  and  transparent  sky  through 
the  greater  part  of  the  day,  Pike's  Peak  would  serve  but 
poorly,  at  least  during  this  season  of  the  year.  As  many  of 
these  storms  were  confined  to  the  immediate,  summit  of  the 
mountain,  a  station  several  thousand  feet  below  would  prob- 
ably offer  more  opportunities  for  work  than  the  peak  itself. 

But  this  is  not  all.  The  definition  of  the  Sun  or  stars  is 
rarely  good  on  Pike's  Peak.  This  is  probably  due,  not  merely 
to  frequent  storms  and  high  wind  velocities,  but  also  in  part 
to  the  fact  that  the  summit  of  the  mountain  is  bare  and  rocky, 
so  that  heated  currents  of  air  rise  from  the  surface  and  ruin 
the  definition  of  the  solar  image.  At  this  altitude  mountain 
sickness  is  also  very  common,  and  would  undoubtedly  inter- 
fere, in  some  degree,  with  the  operation  of  an  observatory. 
The  observers  at  that  time  stationed  there  by  the  Weather 
Bureau  informed  us  that  they  could  not  remain  on  the 
mountain  for  long  periods  without  impairment  of  health  and 
energy.  Two-thirds  of  the  tourists  who  came  to  the  summit, 
by  the  railway  or  on  foot,  were  visibly  affected  by  the  high 
altitude.  Another  cause  of  difficulty  at  the  time  was  forest 
fires  in  the  mountains  surrounding  the  peak,  which  sent 
volumes  of  smoke  into  the  air.  This  rose  to  a  great  altitude 
and  destroyed  the  deep  blue  of  the  sky. 

The  unsuccessful  attempts  to  photograph  the  corona  were 
renewed  on  Mount  Etna  in  July,  1894,  through  the  kindness 
of  Professor  Ricco,  director  of  the  Bellini  Observatories 


ADVANTAGES  OF  HIGH  ALTITUDES  117 

of  Catania  and  Mount  Etna.  Our  party,  consisting  of 
Professor  Ricc6,  Signorina  Ricco,  Antonino  Capra,  mecha- 
nician of  the  observatories,  Mrs.  Hale,  and  myself,  left 
Catania  on  July  7.  After  a  drive  of  three  hours  we  arrived 
at  Nicolosi,  where  we  spent  the  night.  The  following  extracts 
from  my  diary  relate  mainly  to  the  atmospheric  conditions 
encountered : 

July  8.  Left  Nicolosi  at  6  A.  M.  Arrived  at  Casa  del  Bosco 
(4,760  feet)  at  8h  30 m.  Examined  sky  frequently,  and  found  slight 
decrease  of  white  as  we  ascended.  Crossed  lava  stream  of  1892, 
and  had  excellent  view  of  the  craters  of  that  year,  the  latest  of  which 
still  emits  vapor.  Arrived  at  the  observatory  (9,650  feet)  at  lh  35m. 
The  temperature  had  fallen  to  9°  C.,  and  the  sky  was  nearly  covered 
with  clouds.  Half  an  hour  later  we  were  enveloped  in  cloud,  which 
surrounded  us  until  evening,  when  sky  was  whitish,  with  marked 
halo  around  Moon.  Stars  unsteady,  even  in  zenith. 

July  9.  Sky  clear,  with  strong  wind  blowing  the  smoke  from 
the  great  crater  (which  rose  behind  the  observatory  to  an  altitude 
of  10,900  feet)  away  from  the  direction  of  the  Sun.  Half  the  island 
of  Sicily  was  dimly  visible  from  the  observatory  through  a  great 
brown  bank  of  thick  haze,  the  upper  surface  of  which  seemed  to 
be  nearly  on  a  level  with  us.  Cumulus  clouds  commenced  to  form 
at  9h,  and  soon  the  sky  was  nearly  covered.  At  12h  the  Sun  was  seen 
between  passing  clouds  to  be  surrounded  by 'a  bright  halo.  Wind 
changed  to  west  in  the  afternoon,  and  sky  became  much  whiter. 

July  10.  Wind  blew  smoke  of  great  crater  over  Sun,  making 
sky  very  white.  Observed  Sun  with  Professor  Ricc6  by  projection 
with  12-inch  telescope.  Image  rather  better  than  at  Catania,  but 
became  unsteady  later.  At  10 h  some  small  cumulus  clouds  had 
formed,  and  Sun  was  surrounded  by  bright  halo.  Clouds  of  insects 
were  also  noticed  in  direction  of  Sun,  as  on  Pike's  Peak.  Observed 
prominences  with  Professor  Riccb,  but  images  were  no  better  than 
at  Catania.  At  sunset  watched  shadow  of  Etna  from  the  Torre  del 
Filosofo.  Whole  sky  covered  with  dense  haze. 

July  11.  Sky  very  white,  bright  ring  around  Sun.  Observed 
atmospheric  lines  with  direct-vision  spectroscope.  Balanced  tele- 
scope, and  observed  Sun  by  projection.  Seeing  excellent;  granu- 
lation, spots,  and  f  aculae  well  denned.  Strong  odor  of  sulphur.  At 


118  STELLAR  EVOLUTION 

sunset  visited  Valle  del  Bove.  Sky  filled  with  haze,  and  almost  too 
bright  for  the  eye  10°  from  Sun. 

July  12.  Sky  very  white.  Wind  still  blowing  smoke  from  crater 
over  Sun.  Bank  of  haze  above  level  of  observatory.  Observed  Sun 
by  projection  with  Professor  Ricc6;  image  unsteady.  Climbed  to 
top  of  crater,  and  found  sky  in  zenith  of  deeper  blue  than  when  seen 
from  observatory.  Whole  island  enveloped  in  haze.  Descended 
to  observatory  by  moonlight;  double  halo  around  Moon.  Observed 
Moon,  Saturn,  and  several  stars  with  the  12-inch,  using  powers  up 
to  430.  Seeing  magnificent;  images  almost  perfectly  steady  with 
highest  power.  Both  Moon  and  Saturn  were  very  low,  but  images 
were  remarkably  good.  With  naked  eye  scintillation  was  hardly 
perceptible  in  stars  higher  than  30°. 

July  13.  Wind  blowing  from  direction  of  crater,  but  sky  best 
since  July  9:  cloudless  and  generally  whitish,  but  increase  in  bright- 
ness toward  Sun  was  gradual.  Much  dust.  Telescope  in  use  until 
9h  4-Qm  by  Professor  Ricc6  for  daily  record  of  chromosphere.  Prom- 
inences very  well  seen.  At  9h  50m  broad  and  brilliant  ring  of 
whiteness  around  Sun,  making  it  useless  to  try  for  corona.  Smoke 
blowing  directly  over  Sun,  and  diffusing  through  entire  sky.  Solar 
image  observed  by  projection;  definition  very  poor.  At  11 h  sky  had 
improved,  and  preparations  were  made  to  photograph  corona,  but 
five  minutes  later  more  smoke  blew  over  Sun,  and  sky  became  very 
white.  Mirror  found  to  be  dewed,  and  surface  badly  tarnished  by 
the  sulphurous  fumes,  though  it  had  been  tightly  covered  every 
moment  it  was  not  in  use.  Sky  around  Sun  remained  bright,  and 
wind  was  so  violent  that  no  photographs  could  be  made.  Strong 
sulphurous  odor. 

July  14.  Smoke  blowing  across  sun.  Strong  sulphurous  odor. 
Whole  eastern  sky  white.  Prominences  fairly  well  seen  at  7h  45m. 
Left  observatory  at  3  h,  and  arrived  at  Catania  about  midnight. 

As  I  was  assured  by  Tacchini  and  Ricco  that  the  sky  is 
frequently  very  clear  on  Etna,  it  may  safely  be  concluded 
that  the  difficulties  we  encountered  were  exceptional.  During 
the  entire  time  of  our  stay  in  southern  Italy  and  Sicily  the 
atmosphere  was  very  hazy,  and  the  sky  was  rarely  of  a  deep 
blue.  I  was  told  by  Galvagno,  the  custodian  of  the  Etna 
Observatory,  that  the  smoke  this  year  was  much  more  notice- 


ADVANTAGES  OF  HIGH  ALTITUDES  119 

able  than  usual.  If  the  wind  had  blown  it  away  from,  instead 
of  toward,  us,  the  sky  would  probably  have  been  pure,  though 
hardly  as  blue  as  when  seen  from  Pike's  Peak  during  the 
first  part  of  oar  visit  there.1 

So  much  for  the  results  of  brief  personal  experience  in 
Sicily  and  the  Rocky  Mountains.  From  the  standpoint  of 
a  solar  observer  requiring  fine  definition,  they  do  not  appear 
very  encouraging.  Moreover,  conclusions  reached  by  other 
astronomers  have  been  equally  unfavorable  to  Colorado  air; 
and  we  find  Piazzi  Smith,  in  his  book  Teneriffe:  An  Astron- 
omer's Experiment,  reporting  but  very  little  good  solar 
definition  at  altitudes  up  to  10,700  feet  on  a  tropical  island. 
His  expedition  to  Teneriffe  in  1856,  made  for  the  express 
purpose  of  testing  the  atmospheric  conditions  on  a  mountain- 
peak,  was  the  first  serious  study  of  this  kind.  The  trans- 
parency of  the  air  and  the  definition  of  the  stars  by  night 
were  found  to  be  excellent;  but  high  winds,  dust  in  the 
apper  atmosphere,  and  unsteady  solar  images  were  also 
encountered. 

However,  good  solar  definition  is  experienced  on  Mont 
Blanc  (15,780  feet),  at  the  Kodaikanal  Solar  Observatory 
in  India  (7,700  feet),  and  at  the  Pic-du-Midi  in  France. 
There  is  obviously  no  incompatibility  between  high  altitudes 
and  good  solar  definition.  The  poor  definition  reported  by 
various  observers  on  mountain-peaks  is  due  either  to  the 
prevalence  of  storms  or  to  local  disturbances,  caused  by 
warm  air  rising  from  the  heated  summits  of  mountain-tops 
protected  by  little  or  no  foliage.  At  Mount  Hamilton,  where 
the  night  conditions  are  so  favorable,  the  slopes  immediately 
around  the  summit  are  composed  of  bare  rock,  which  becomes 
intensely  heated  and  necessarily  affects  the  solar  definition. 
This  is  a  matter  of  no  special  consequence  to  the  Lick 

iThe  attempts  to  photograph  the  corona  were  continued  by  Ricc6  under  better 
conditions,  but  neither  this  method  nor  any  other  has  yet  proved  successful. 


120  STELLAR  EVOLUTION 

Observatory  (Plate  XL VIII),  since  the  work  is  confined  to 
night  observations.  The  great  number  of  admirable  results, 
many  of  them  requiring  the  finest  definition,  which  have 
been  obtained  at  the  Lick  Observatory,  afford  the  best  of 
evidence  that  its  site  was  well  chosen. 

The  results  of  experience  in  various  parts  of  the  world 
would  seem  to  indicate  that  a  mountain  observatory,  if  it  is 
to  enjoy  good  conditions  both  by  night  and  by  day,  should 
be  situated  in  a  climate  where  the  sky  is  clear  continuously 
for  periods  of  several  weeks  or  months,  and  the  average  wind 
velocity  is  low.  The  summit  of  the  mountain,  as  well  as  its 
slopes,  should  be  covered  with  foliage,  to  protect  it  from  the 
heat  of  the  Sun.  Finally,  the  elevation  should  be  sufficient 
to  escape  the  dust  which  diffuses  itself  through  the  air  in 
the  dry  season,  and  the  low-lying  fogs  and  clouds  frequently 
encountered  in  regions  near  the  sea. 


CHAPTER  XIV 
THE  MOUNT  WILSON  SOLAR  OBSERVATORY 

FEOM  the  preceding  chapters,  it  will  be  seen  how  the  plan 
of  research  of  the  Solar  Observatory  was  developed.  At 
Kenwood  a  programme  of  solar  observations,  involving  the 
use  of  the  spectroheliograph,  the  photographic  study  of  the 
spectra  of  Sun-spots  and  other  solar  phenomena,  and  the  fullest 
possible  application  of  laboratory  methods  in  astrophysical 
research,  was  instituted.  At  the  Yerkes  Observatory  this 
programme  was  broadened  and  extended,  in  the  hope  of 
providing  ultimately  for  the  general  study  of  stellar  evolu- 
tion; the  possibilities  of  the  spectroheliograph  were  more 
fully  realized,  through  the  advantages  offered  by  the  40-inch 
refractor;  and  instruments  better  adapted  than  the  large 
refractor  for  the  further  prosecution  of  the  work,  such  as  the 
Snow  telescope  for  solar  research,  and  the  60-inch  reflector 
for  stellar  investigations,  were  designed  and  partially  or 
wholly  constructed.  After  this  period  of  preparation,  devoted 
in  large  part  to  the  development  of  plans  and  methods,  the 
Mount  Wilson  Solar  Observatory  was  organized  for  the  study 
of  stellar  evolution,  at  a  station  enjoying  the  best  climatic 
advantages. 

In  brief,  the  scheme  of  research  of  the  Solar  Observatory 
comprises:  (1)  solar  investigations,  to  contribute  toward  our 
knowledge  of  the  Sun  (a)  as  a  typical  star  and  (6)  as  the 
central  body  of  the  solar  system;  (2)  photographic  and 
spectroscopic  studies  of  stars  and  nebulae,  bearing  directly 
upon  the  physical  nature  of  these  bodies,  with  special  refer- 
ence to  their  development;  (3)  laboratory  investigations,  for 
the  interpretation  of  solar  and  stellar  phenomena.  With 

121 


122  STELLAR  EVOLUTION 

the  central  problem  in  mind,  each  successive  research  is 
designed  to  occupy  a  logical  place  in  a  concentrated  attack, 
proceeding  along  these  converging  lines. 

The  variety  of  the  problems  connected  with  the  establish- 
ment of  the  Solar  Observatory  on  Mount  Wilson  affords  a 
good  illustration  of  the  diversified  work  of  an  astronomer. 
It  was  necessary,  in  the  first  place,  to  test  the  atmospheric 
conditions  by  means  of  telescopic  and  meteorological  observa- 
tions extending  over  a  considerable  period  of  time,  in  order 
to  make  certain  that  the  site  would  prove  suitable.  In  the 
second  place,  since  the  summit  could  be  reached  only  by  a 
narrow  mountain  trail,  it  was  evident  from  the  outset  that 
the  question  of  transporting  building  materials  and  the  parts 
of  heavy  instruments  would  not  be  an  easy  one  to  solve. 
Again,  since  one  of  the  prime  purposes  of  the  new  observatory 
was  to  take  advantage  of  the  possibilities  of  improved  instru- 
ments, the  design  and  construction  of  the  telescopes,  spectro- 
scopes, and  other  appliances  would  require  the  solution  of 
many  instrumental  and  engineering  problems,  and  much  work 
of  experiment.  It  was  known,  for  example,  that  glass  mirrors 
change  their  form  decidedly  when  exposed  to  the  Sun's  rays. 
For  this  reason  it  was  to  be  feared  that  they  might  not  give 
good  solar  images.  This  is  a  matter  of  fundamental  impor- 
tance, since  the  fixed  telescope  for  solar  observations  neces- 
sarily involves  the  employment  of  mirrors.  In  addition  to 
these  questions,  many  others,  very  diverse  in  character,  pre- 
sented themselves.  These  included  the  preparation  of  a 
programme  of  research,  adapted  for  the  special  requirements 
of  the  new  observatory,  in  which  all  the  investigations  in 
progress  were  to  be  closely  correlated ;  the  consideration  of 
the  best  methods  of  discussing  and  interpreting  the  photo- 
graphs made  with  the  spectroheliograph  and  other  instru- 
ments; the  invention  and  construction  of  special  measuring 
and  computing  machines,  etc. 


MOUNT  WILSON  SOLAE  OBSERVATORY         123 

From  a  meteorological  standpoint,  the  state  of  California 
may  be  divided  into  three  parts.  In  the  northern  region 
the  rainfall  is  very  considerable,  much  cloudiness  prevails, 
and  in  almost  all  respects  the  conditions  are  unfavorable  for 
astronomical  work.  The  central  region,  which  may  be  con- 
sidered to  extend  as  far  south  as  Point  Conception,  is  favored 
with  much  better  weather  conditions,  best  exemplified  at  the 
Lick  Observatory,  on  Mount  Hamilton,  where  a  high  average 
of  night-seeing  is  maintained  during  a  large  part  of  the  year. 
In  the  southern  part  of  California  the  climatic  conditions  are 
different  from  those  which  prevail  in  the  two  other  sections 
of  the  state.  The  lighter  rainfall  is  naturally  associated  with 
fewer  clouds,  a  remarkably  steady  barometer,  and  very  light 
winds. 

There  can  be  no  doubt  that  the  character  of  the  country 
immediately  adjoining  an  observatory  site  affects  the  condi- 
tions for  astronomical  work  to  an  important  degree.  For  this 
reason  it  became  desirable  to  make  preliminary  tests  of  a  con- 
siderable number  of  points  in  southern  California.  Similar 
tests  might  have  been  desirable  in  Arizona,  were  it  not  for 
the  thunderstorms  that  prevail  during  the  summer  months  in 
the  vicinity  of  Flagstaff,  and  other  promising  localities,  which 
would  interfere  so  seriously  with  solar  work  as  to  put  this 
region  almost  entirely  out  of  consideration.  As  there  were 
other  serious  objections  to  Arizona  sites,  and  as  Hussey's  tests 
at  Flagstaff  did  not  indicate  that  the  conditions  were  as  favor- 
able as  in  California,  attention  was  concentrated  on  the  rela- 
tive claims  of  various  mountains  in  southern  California. 

Hussey's  tests  in  this  region  included  Echo  Mountain, 
Mount  Lowe,  and  Mount  Wilson,  in  the  Sierra  Madre  range, 
and  Cuyamaca  and  Palomar,  much  farther  to  the  south. 
His  observations  seemed  to  leave  no  doubt  that  Mount  Wilson 
would  prove  to  be  the  best  site  for  the  purposes  of  a  solar 
observatory. 


124  STELLAR  EVOLUTION 

Mount  Wilson  is  one  of  many  mountains  that  form  the 
southern  boundary  of  the  Sierra  Madre  range  (Plate  XLIX). 
Standing  at  a  distance  of  thirty  miles  from  the  ocean,  it  rises 
abruptly  from  the  valley  floor,  flanked  only  by  a  few  spurs  of 
lesser  elevation,  of  which  Mount  Harvard  is  the  highest. 
Except  for  a  narrow  saddle,  Mount  Wilson  is  separated  from 
Mount  Harvard  by  a  deep  canon,  the  walls  of  which  are  very 
precipitous.  Farther  to  the  west,  beyond  the  saddle  leading 
to  Mount  Harvard,  the  ridge  of  Mount  Wilson  forms  the  upper 
extremity  of  Eaton  Canon,  which  leads  directly  to  the  San 
Gabriel  Valley.  East  and  north  of  Mount  Wilson  lies  the 
deep  canon  through  which  flows  the  west  fork  of  the  San 
Gabriel  River,  and  beyond  this  rise  a  constant  succession  of 
mountains,  most  of  them  higher  than  Mount  Wilson,  which 
extend  in  a  broken  mass  to  the  Mojave  Desert.  The  Sierra 
Madre  range  forms  the  northern  boundary  of  the  San 
Gabriel  Valley,  which  is  further  protected  toward  the  east 
from  the  desert  by  the  high  peaks  of  the  San  Bernardino 
range. 

The  view  from  the  summit  of  Mount  Wilson  is  most  exten- 
sive, embracing  the  whole  of  southern  California,  and  reach- 
ing out  over  the  Pacific  to  islands  nearly  one  hundred  miles 
distant.  Cuyamaca,  about  130  miles  to  the  south,  not  far 
from  the  Mexican  boundary,  is  easily  visible.  San  Bernardino 
and  San  Jacinto  peaks,  the  latter  90  miles  away,  are  so  dis- 
tinctly seen  under  normal  conditions  that  a  station  might 
easily  be  established  on  either  of  them,  for  experiments  in 
measuring  the  velocity  of  light  from  Mount  Wilson.  Mount 
San  Antonio  (10,080  feet),  25  miles  away,  has  already  served 
as  a  station  for  certain  observations  of  the  solar  radiation, 
supplementing  the  work  of  the  Smithsonian  Expedition  at 
Mount  Wilson  (Plate  L). 

During  a  part  of  the  year,  particularly  from  April  to 
August,  fog  rolls  in  from  the  ocean  and  covers  much  of  the 


MOUNT  WILSON  SOLAK  OBSEKVATORY         125 

San  Gabriel  Valley  during  the  night  (  Plate  LI).  But  these 
fog-clouds  rarely  attain  elevations  exceeding  3,000  feet.  The 
mountains  of  the  Sierra  Madre  range  rise  high  above  the 
fog,  and  during  many  months  of  the  year  they  enjoy  practi- 
cally continuous  sunshine.  In  summer  the  sea  breeze  blows 
for  a  part  of  the  day,  but  it  attains  only  a  low  velocity, 
which  decreases  in  passing  from  the  valley  to  the  moun- 
tain tops. 

Mount  Wilson  is  reached  from  the  San  Gabriel  Valley  by 
either  one  of  two  trails.  One  of  these,  known  as  the  "Wilson 
Trail,"  ascends  from  Sierra  Madre,  and  is  steep  and  irregular. 
The  other,  called  the  "New  Trail,"  rises  from  the  foot  of 
Eaton  Canon,  about  6^  miles  from  Pasadena,  and  is  about 
9J  miles  long.  When  our  work  commenced,  it  was  but  little 
over  two  feet  in  width  at  its  narrowest  parts.  It  has  an  average 
grade  of  about  10  per  cent.,  and  is  much  better  adapted  for 
transportation  purposes  than  the  old  Wilson  Trail. 

Some  hundreds  of  tons  of  building  material  for  the 
observatory  have  been  taken  over  the  New  Trail,  on  the  backs 
of  mules  or  "burros"  (donkeys)  (Plate  LII).  The  heavier 
parts  of  instruments,  which  could  not  be  taken  up  in  this 
way,  were  carried  on  a  special  truck  built  for  the  purpose 
(Plate  LIII) .  The  running-gear  consists  of  four  automobile 
wheels  with  rubber  tires.  The  body  of  the  truck  is  hung  by 
wrought-iron  yokes  from  the  running-gear,  with  its  lower 
surface  at  a  height  of  only  six  inches  above  the  ground. 
Steering-gear,  of  the  type  used  on  automobiles,  is  provided 
for  both  pairs  of  wheels.  A  man  riding  on  the  load  steers 
the  forward  wheels,  while  the  rear  wheels  are  steered  with  a 
tiller  by  a  man  walking  behind  the  carriage.  A  single  large 
horse  pulls  a  load  of  a  thousand  pounds  on  this  carriage  with- 
out difficulty.  With  two  horses,  used  in  relays,  the  trip  from 
the  lower  end  of  the  trail  to  the  summit  and  return  is  com- 
pleted with  such  a  load  in  about  fifteen  hours.  About  sixty 


126  STELLAR  EVOLUTION 

round  trips  were  made  with  this  truck  for  the  purpose  of 
carrying  the  mirrors,  lenses,  and  heavy  castings  of  the  Snow 
and  Bruce  telescopes,  the  parts  of  a  15-H.  P.  gas  engine, 
and  other  heavy  machines,  as  well  as  the  4-inch  pipe  columns 
used  in  constructing  the  steel  skeleton  of  the  Snow  telescope 
house. 

During  the  first  two  years,  it  was  hoped  that  a  railway 
would  be  constructed  to  the  summit  of  the  mountain,  where 
a  hotel  had  already  been  erected.  When  it  finally  appeared 
that  this  hope  must  be  abandoned,  we  were  compelled  to 
adopt  the  alternative  of  widening  the  New  Trail  into  a  wagon- 
road  (Plate  LIV).  This  work,  which  was  done  during  the 
autumn  and  spring  of  1906  and  1907,  was  considerably  ham- 
pered by  unprecedented  storms  in  December  and  January. 
The  snow  on  the  summit  of  Mount  Wilson  (Plate  LV)  was 
five  feet  deep  on  a  level,  and  the  torrential  rains,  below  the 
snow  line,  brought  down  thousands  of  tons  of  earth  and  rocks 
from  the  steep  slopes  of  the  mountain.  When  these  difficul- 
ties had  been  overcome,  the  transportation  problem  was  so 
far  solved  as  to  permit  the  structural  steel  for  the  building 
and  dome  of  the  60-inch  reflector  to  be  hauled  to  their 
destination. 

Our  systematic  tests  of  the  atmospheric  conditions  on 
Mount  Wilson  began  in  March,  1904.  An  old  log  cabin, 
which  had  been  in  a  state  of  partial  ruin,  was  rendered 
habitable  and  occupied  until  the  "Monastery"  was  com- 
pleted, in  the  following  December.  Frequent  tests  of  the 
solar  definition  were  made  with  a  3J-inch  refracting  tele- 
scope, supplemented  by  meteorological  observations. 

The  specific  requirements  of  a  site  for  an  observatory  to 
be  devoted  to  solar  research  and  the  study  of  stellar  evolu- 
tion are  as  follows: 

1.  Excellent  definition  of  the  solar  image,  on  many  days 
of  the  year. 


MOUNT  WILSON  SOLAR  OBSERVATORY         127 

2.  Excellent  definition  by  night,  so  as  to  permit  reflecting 
telescopes  of  large  aperture  to  be  used  for  the  most  exacting 
work. 

3.  Great  transparency  of  the  day  and  night  sky,  essential 
for   accurate   determinations   of    the   "solar  constant"   (the 
total  heat  radiation  of  the  Sun,  at  a  point  outside  of  the 
Earth's   atmosphere),    and    the    photography  of    stars    and 
nebulae  requiring  very  long  exposures. 

4.  Continuous  clear  weather  for  periods  of  many  weeks, 
rendering  possible  daily  observations  of  changing  phenom- 
ena, of  which  an  imperfect  or  erroneous  idea  might  be  derived 
from  scattered  observations. 

5.  A  low  average   wind  velocity,  especially  during   the 
best  observing  season,  to  insure  freedom  from  vibration  of 
telescopes  employed  for  photographic  work. 

It  is  easy  to  see  why  the  definition  of  the  Sun's  image  is 
usually  much  inferior  to  that  of  the  stars  or  planets.  The 
heating  of  the  earth,  caused  by  the  Sun's  rays,  produces 
currents  of  warm  air,  which  rise  and  mix  with  the  cooler  air 
above.  It  has  already  been  explained  that  poor  definition  is 
produced  by  irregular  refraction  in  the  atmosphere,  and  that 
this  is  caused  by  irregularities  in  the  temperature  of  the  air 
through  which  the  light  rays  pass.  In  this  respect  a  moun- 
tain peak  may  have  some  disadvantages  as  compared  with  an 
extensive  level  area,  because  the  rising  currents  of  warm  air 
follow  the  mountain  sides  and  tend  to  produce  marked  dis- 
turbances in  the  images  observed  from  the  summit.  It  is 
evident  that  this  effect  will  be  greatly  enhanced  if  the  moun- 
tain is  bare  and  rocky,  instead  of  having  its  slopes  covered 
with  trees  and  bushes.  As  the  latter  condition  prevails  on 
most  of  the  slopes  of  Mount  Wilson,  the  heating  of  the  air 
is  much  less  pronounced  than  in  the  case  of  many  other 
mountains.  It  is  nevertheless  very  noticeable,  and  for  this 
reason  the  best  observations  of  the  Sun  are  made  one  or  two 


128  STELLAR  EVOLUTION 

hours  after  sunrise,  and  about  the  same  time  before  sunset. 
It  is  true  that  great  depths  of  atmosphere  must  be  traversed 
by  the  Sun's  rays  when  it  is  so  near  the  horizon.  Neverthe- 
less, the  image  on  a  large  number  of  days  in  the  summer 
season  is  wonderfully  sharp  and  distinct,  permitting  the 
finest  details  of  structure  to  be  observed.  The  conclusions 
based  upon  observations  made  with  the  3J-inch  refractor 
were  afterward  confirmed  with  the  large  aperture  of  the 
Snow  telescope,  leaving  no  doubt  that  with  respect  to  solar 
definition  Mount  Wilson  offers  very  exceptional  advantages. 

The  tests  of  the  night  definition,  and  of  the  transparency 
of  the  night  sky,  were  made  by  Barnard,  during  the  work 
of  the  Hooker  Expedition.  In  chap,  v  a  description  has 
been  given  of  the  Bruce  10-inch  photographic  telescope 
of  the  Yerkes  Observatory,  used  by  Barnard  in  his  studies 
of  the  Milky  Way.  In  order  to  extend  farther  south  the 
work  previously  done  with  an  instrument  of  6  inches  aper- 
ture on  Mount  Hamilton,  Barnard  brought  the  Bruce  tele- 
scope to  Mount  Wilson  and  made  with  it  a  remarkable  series 
of  photographs.  Mount  Wilson  (latitude +  34°  13 ')  is  8° 
south  of  the  Yerkes  Observatory,  and  3°  south  of  the  Lick  Ob- 
servatory. This  fact,  combined  with  the  great  transparency 
of  the  sky,  permitted  Barnard  to  photograph  regions  .of  the 
Milky  Way  which  had  been  out  of  reach  in  his  earlier  work. 

The  best  way  of  comparing  the  transparency  of  the  sky 
at  Lake  Geneva  and  Mount  Wilson  is  by  taking  two  photo- 
graphs of  the  same  region  of  the  heavens,  with  the  same 
exposure  time,  on  photographic  plates  of  the  same  sensitive- 
ness, used  with  the  same  telescope,  by  the  same  observer. 
Such  a  comparison  is  illustrated  in  Plate  LVI,  which  repre- 
sents the  cluster  Messier  35.  The  difference  in  the  number  of 
stars  included  on  the  photograph  is  a  striking  illustration  of 
the  advantages  of  Mount  Wilson.  Indeed,  if  this  result  were 
not  confirmed  by  many  others,  and  regarded  by  Barnard 


MOUNT  WILSON  SOLAR  OBSERVATORY         129 

as  representing  a  fair  relative  test,  it  might  be  supposed 
that  some  difference  in  the  mode  of  development  or  in  the 
sensitiveness  of  the  plate  had  entered.  The  night  on  which 
the  Mount  Wilson  photograph  was  made  was  an  average  sum- 
mer night,  while  in  the  case  of  the  Yerkes  Observatory 
photograph  the  transparency  was  possibly  higher  than  the 
average  there. 

An  illustration  of  the  same  sort  is  given  in  Plate  LVII, 
which  shows  the  Pleiades  as  photographed  by  Barnard  with 
the  Bruce  telescope,  with  an  exposure  of  3  hours  and  48 
minutes  at  Mount  Wilson  and  9  hours  and  47  minutes  at 
the  Yerkes  Observatory.  It  will  be  seen  that  the  first  photo- 
graph shows  quite  as  many  stars  as  the  second,  and  also 
has  a  great  advantage  in  sharpness,  as  indicated  by  the 
much  larger  amount  of  detail  brought  out  in  the  nebulae. 
This  is  due  to  the  fact  that  the  greater  diffusion  of  light  in 
the  Wisconsin  sky  tends  to  obliterate  the  finer  details  of  the 
photograph.  It  is  interesting  to  conjecture  what  advantages 
will  result  from  the  use  of  the  60-inch  reflector  under  these 
fine  conditions. 

During  the  long  exposures  Barnard  kept  a  star  on  a  pair 
of  cross-hairs  in  the  eye-piece  of  a  5-inch  refractor,  attached 
to  the  Bruce  telescope.  In  this  way  he  observed  the  defini- 
tion of  the  stellar  images  on  a  large  number  of  nights.  As 
previously  explained,  the  definition  of  a  star  does  not  depend 
in  any  considerable  degree  upon  the  transparency  of  the 
atmosphere,  but  rather  upon  the  absence  of  irregular  refrac- 
tion. Barnard  found  the  average  night  "seeing"  to  be 
remarkably  good,  and  this  conclusion  has  also  been  con- 
firmed with  the  large  aperture  of  the  Snow  telescope. 

The  transparency  of  the  sky  by  day  has  been  most 
thoroughly  tested  by  Abbot,  in  his  studies  of  the  "solar 
constant"  of  radiation,  which  are  described  in  chap.  xxii. 
As  compared  with  Washington,  where  the  previous  work 


130  STELLAR  EVOLUTION 

of  the  Smithsonian  Astrophysical  Observatory  has  been 
done,  the  advantages  of  Mount  Wilson  are  very  marked. 
Of  equal  importance  for  this  work  is  the  fact  that  the 
observations  can  be  made  day  after  day,  with  practically  no 
interruption,  for  a  period  of  many  weeks.  In  Washington, 
during  the  same  period,  it  might  be  possible  to  obtain  only 
two  or  three  trustworthy  determinations.  Thus  the  manner 
in  which  the  solar  radiation  varies  can  be  shown,  in  the  one 
case,  by  its  daily  fluctuations,  while  in  the  other  it  might  be 
wholly  concealed. 

Finally,  the  average  wind  velocity  in  the  dry  season 
proved  to  be  extraordinarily  low,  not  only  for  an  exposed 
mountain-peak,  but  as  compared  with  a  station  at  any  level. 
During  the  rainy  season,  when  there  is  much  cloudy  weather, 
violent  storms,  accompanied  by  high  winds,  are  not  uncom- 
mon. But  in  the  dry  season  an  almost  dead  calm  frequently 
prevails  at  night,  and  also  during  the  early  morning  solar 
observations.  In  the  later  hours  of  the  day  there  is  usually 
a  light  breeze.  The  typical  condition  on  Mount  Wilson 
during  the  dry  season  may  be  described  as  a  perfectly  cloud- 
less sky,  and  so  little  breeze  that  the  leaves  are  hardly 
stirred  by  it. 

It  would  be  tedious  to  discuss  the  other  conditions,  such 
as  the  heavy  growth  of  foliage,  the  presence  of  abundant 
springs  of  water,  the  neighborhood  of  large  cities,  etc.,  which 
contribute  toward  the  advantages  of  Mount  Wilson  as  an 
observatory  site.  The  astronomical  tests  have  been  described 
in  detail  because  they  illustrate  the  practical  bearing  of 
atmospheric  conditions  on  astronomical  observations. 


CHAPTER  XV 
THE  SNOW  TELESCOPE 

LEON  FOUCAULT  appears  to  have  been  the  first  to  appre- 
ciate the  advantages  of  a  fixed  telescope,  capable  of  forming 
a  solar  or  stellar  image  within  a  laboratory.  A  large  sidero- 
stat,  constructed  by  Eichens  after  his  designs,  was  completed 
in  1868,  the  year  of  Foucault's  death.  It  remained  at  the 
Paris  Observatory,  where  it  was  subsequently  employed  by 
Deslandres  for  solar  photography.  For  small  images  of  the 
Sun  this  instrument  gave  good  results,  although  the  imper- 
fection of  the  driving  caused  the  image  to  wander  more  or 
less  from  a  fixed  position.  This  difficulty  has  been  inherent 
in  almost  all  types  of  fixed  telescopes.  It  is  coupled  with 
the  inconvenience  that  the  solar  image  produced  by  the 
siderostat  or  heliostat  rotates  in  an  irregular  manner,  which 
would  cause  distortion  in  long-exposure  photography  with  a 
fixed  spectroheliograph. 

It  is  not  easy  to  see  why  the  heliostat,  in  some  of  its 
forms,  was  not  more  rapidly  developed.  With  a  few  excep- 
tions, its  practical  application  has  been  confined  to  small 
heliostats  of  various  types,  used  to  reflect  a  beam  of  sunlight 
into  the  laboratory,  but  not  to  produce  a  large  image  of  the 
Sun.  In  other  words,  the  heliostat  was  not  developed  into 
an  instrument  of  precision,  capable  of  giving  a  large  and 
well-defined  solar  image,  and  maintaining  it  accurately  fixed 
in  position,  until  the  coelostat  was  revived  for  eclipse  pur- 
poses, about  ten  years  ago.1  This  instrument  had  been 
invented  long  before,  but  its  great  advantages,  due  to  the 

!The  great  equatorial  coud&  of  the  Paris  Observatory  is  an  admirable  example 
of  a  fixed  telescope,  but  I  do  not  think  it  has  been  tested  for  solar  observations. 

131 


132  STELLAR  EVOLUTION 

simplicity  of  its  construction,  the  ease  of  driving  it  with  a 
precision  as  great  as  in  the  case  of  the  equatorial  refractor, 
and,  above  all,  the  fact  that  the  solar  image  produced  by  it 
does  not  rotate j  had  been  overlooked.  At  Turner's  sugges- 
tion it  was  employed  for  eclipse  purposes,  at  first  by  some 
of  the  English  parties,  and  subsequently  by  astronomers  in 
all  parts  of  the  world. 

However,  the  conditions  under  which  eclipse  observations 
are  made  are  very  different  from  those  that  obtain  in  ordi- 
nary solar  work.  A  defect  of  the  coelostat  is  that  the  direc- 
tion of  the  beam  of  light  reflected  horizontally  from  the 
mirror  varies  with  the  declination  of  the  Sun.  During  the 
few  minutes  of  a  total  eclipse  the  Sun's  declination  does  not 
change  appreciably,  and  the  telescope  into  which  the  light 
is  reflected  by  the  coelostat  stands  fixed  in  position.  But 
in  solar  observations  continued  throughout  the  year  the 
direction  of  the  reflected  beam  is  constantly  changing,  as 
the  Sun  moves  north  or  south  of  the  equator.  As  it  would 
be  inconvenient  to  swing  the  long  telescope  tube,  which  is 
pointed  at  the  coelostat,  around  through  the  necessarily  large 
angle,  a  second  mirror  must  be  introduced  to  receive  the 
light  reflected  from  the  coelostat  mirror  and  send  it  in  any 
desired  direction.  About  once  a  week,  or  sufficiently  often 
to  cause  no  appreciable  loss  of  sunlight,  the  second  mirror 
is  moved  a  short  distance,  so  that  it  may  continue  to  receive 
all  the  light  of  the  reflected  beam,  in  the  changed  position 
given  it  by  the  variation  in  the  Sun's  declination. 

Another  difficulty  of  the  coelostat,  which  is  common  to 
all  forms  of  heliostat,  but  plays  no  part  in  total-eclipse  work, 
is  the  distortion  of  the  mirrors  by  sunlight.  This  obstacle 
is  really  the  only  serious  one  presented  by  this  form  of 
telescope.  How  it  has  been  met  at  the  Solar  Observatory 
is  explained  below. 

The  Snow  telescope,  the  optical  and  mechanical  parts  of 


THE  SNOW  TELESCOPE 


133 


which  were  constructed  under  Hitch ey's  supervision  in  the 
shops  of  the  Yerkes  Observatory,  is  illustrated  in  Plate 
LVIII.  This  photograph  shows  the  coelostat  and  the  adjust- 
able second  mirror,  whence  the  light  is  reflected  to  a  concave 
mirror  of  60  feet  focal  length,  which  forms  the  solar  image. 
The  general  arrangement  of  the  telescope,  as  established  on 
Mount  Wilson,  is  indicated  in  Fig.  5.  The  coelostat  stands 


FIG.  5 
Plan  and  Elevation  of  Snow  Telescope  House  on  Mount  Wilson 


on  a  carriage,  which  can  be  moved  east  or  west  along  the 
line  aa.  On  account  of  the  configuration  of  the  ground, 
which  falls  rapidly  toward  the  north,  it  was  necessary  to 
make  the  long  axis  of  the  building  run  15°  east  of  north, 
instead  of  being  exactly  in  the  meridian.  For  the  same 
reason  this  axis  is  not  horizontal,  but  inclined  downward  5° 
toward  the  north.  Without  these  adaptations  of  the  plan  to 
the  conditions  of  the  site,  the  height  of  the  northern  part  of 
the  building  would  have  been  very  great,  involving  serious 
increase  of  expense.  The  rails  66,  on  which  the  carriage 
bearing  the  second  mirror  slides,  are  parallel  to  the  optical 
axis.  The  coelostat  mirror,  30  inches  in  diameter,  and  the 
second  mirror,  24  inches  in  diameter,  have  plane  surfaces, 


134  STELLAE  EVOLUTION 

and  serve  merely  for  bringing  the  sunlight  into  the  telescope 
house.  The  plane  of  the  coelostat  mirror  is  parallel  to  the 
Earth's  axis,  and  the  mirror  can  be  rotated  around  this  axis 
once  in  forty-eight  hours,  by  means  of  a  driving-clock.  This 
exactly  counteracts  the  motion  of  the  beam  due  to  the  Sun's 
apparent  motion  through  the  heavens. 

From  the  second  mirror  the  light  passes  to  either  one  of 
two  concave  mirrors,  each  24  inches  in  diameter  (Plate  LIX) . 
One  of  these,  which  has  a  focal  length  of  60  feet,  is  supported 
on  a  carriage  so  that  it  can  be  moved  (for  focusing)  along 
the  rails  cc,  which  are  mounted  on  the  small  pier  shown  near 
the  middle  of  Fig.  5.  This  mirror  produces  an  image 
of  the  Sun  about  6.7  inches  in  diameter,  at  a  position  in  the 
spectroscope  house  determined  by  the  angle  which  the  con- 
cave mirror  makes  with  the  incident  beam  of  sunlight.  If 
the  mirror  stood  normal  to  the  beam,  the  sunlight  would  be 
reflected  directly  back  upon  itself  toward  the  second  mirror. 
If,  however,  the  concave  mirror  is  turned  slightly  to  one  side, 
the  solar  image  can  be  formed  at  the  end  of  the  pier  /,  where 
the  5-foot  spectroheliograph  stands.  By  moving  the  mirror 
back  toward  the  north,  along  the  rails  on  which  it  slides,  the 
image  can  be  brought  to  a  focus  on  the  pier  t,  where  the  slit 
and  photographic  plate  of  a  Littrow  spectrograph,  of  18  feet 
focal  length,  are  mounted.  Again,  by  rotating  the  concave 
mirror  so  as  to  return  the  beam  in  a  somewhat  different  direc- 
tion, the  solar  image  can  be  sent  into  the  constant  tempera- 
ture room  III,  where  the  bolographic  apparatus,  for  studying 
the  heat  radiation  of  different  parts  of  the  Sun,  is  mounted 
on  the  massive  triangular  pier  kick. 

If  a  larger  solar  image  is  required,  the  mirror  of  60  feet 
focal  length  is  moved  out  of  the  way,  and  the  beam  from  the 
second  mirror  allowed  to  pass  to  a  concave  mirror  of  143 
feet  focal  length,  mounted  on  a  pier  at  the  extreme  north 
end  of  the  telescope  house.  The  image  of  the  Sun  is  then 


THE  SNOW  TELESCOPE  135 

formed  at 'the  pier  g,  143  feet  from  the  concave  mirror. 
This  image  is  16  inches  in  diameter,  and  is  used  for  the 
special  study  of  solar  details,  for  which  a  large  scale  is 
required. 

The  remarkable  convenience  of  such  a  telescope,  when 
contrasted  with  a  great  movable  refractor  like  the  Yerkes 
telescope,  is  immediately  evident.  Instead  of  attaching  each 
heavy  instrument,  one  by  one,  to  the  end  of  a  moving  tele- 
scope tube,  it  is  set  up  once  for  all  on  a  pier,  where  its 
adjustments  need  never  be  disturbed.  It  is  thus  possible  to 
pass  rapidly  from  one  instrument  to  another,  photographing 
the  forms  of  the  calcium  flocculi,  for  example,  with  the 
spectroheliograph,  and  their  spectra,  only  a  moment  later, 
with  the  powerful  Littrow  spectrograph.  In  view  of  the 
importance  of  studying  solar  phenomena  nearly  simulta- 
neously by  various  methods,  and  of  closely  correlating  the 
observations,  the  advantages  afforded  by  such  a  telescope 
will  be  easily  recognized. 

The  peculiar  form  of  house  in  which  the  Snow  telescope 
is  mounted  calls  for  a  word  of  explanation.  In  previous 
experiments,  some  of  which  were  made  on  Mount  Wilson  in 
the  spring  of  1904,  the  conclusion  was  reached  that  the  dis- 
turbance of  the  definition  caused  by  warm  air  rising  from 
the  ground  in  the  immediate  neighborhood  of  the  heliostat 
could  be  appreciably  reduced  by  mounting  the  instrument 
at  a  considerable  height.  Observations  made  with  a  tele- 
scope supported  in  a  tree,  at  various  heights  up  to  seventy 
feet,  seemed  to  leave  no  doubt  regarding  this  point.  A 
second  consideration,  the  importance  of  which  had  been 
particularly  emphasized  by  experience  with  a  smaller  coelo- 
stat  telescope,  having  a  closed  tube  not  provided  with  means 
of  ventilation,  was  the  necessity  of  designing  a  house  so  that 
the  temperature  within  would  be  at  all  times  as  nearly  as 
possible  the  same  as  that  of  the  outer  air.  It  is  evident  that, 


136  STELLAK  EVOLUTION 

if  this  condition  is  not  met,  the  mixture  of  air  of  different 
temperatures  at  the  open  end  of  the  house,  through  which 
the  beam  enters,  will  cause  irregular  refraction  and  conse- 
quent disturbance  of  the  image, 

Plate  LX  shows  the  pier  on  which  the  coelostat  is 
mounted,  at  a  height  of  nearly  30  feet  above  the  ground. 
Since  the  parallel  rays  from  the  coelostat  to  the  concave 
mirror  pass  through  a  closed  house,  it  is  not  essential  that  that 
part  of  the  building  should  stand  high  above  the  ground.  It 
is  important,  however,  that  disturbances  due  to  heating  of 
the  walls,  caused  by  sunlight  falling  upon  them,  be  obviated. 
For  this  reason  all  parts  of  the  building,  including  the 
movable  shelter,  the  spectroscopic  laboratory,  and  the  long 
narrow  house  extending  north  from  the  laboratory,  have  an 
inner  wall  and  ceiling  of  canvas,  and  an  outer  wall  composed 
of  canvas  louvers,  very  completely  ventilated.  The  roof  is 
also  ventilated,  by  wooden  louvers  at  the  ridge  throughout 
the  entire  length  of  the  movable  shelter  and  the  north  exten- 
sion, and  at  the  peak  of  the  laboratory.  Rain  and  snow  are 
prevented  from  entering  the  roof  louvers  by  means  of  canvas 
guards,  which  can  be  raised  or  lowered  at  will.  The  house 
extending  north  from  the  laboratory  has  a  floor  of  canvas, 
with  a  space  below,  through  which  the  air  may  pass  freely. 

The  louvers  surrounding  the  coelostat  pier  are  intended 
to  protect  the  pier  from  vibration  caused  by  the  wind,  and 
from  heating  by  the  Sun.  The  steel  structure  does  not  touch 
the  pier  at  any  point,  and  is  therefore  made  rigid  enough  to 
support  itself  in  high  winds.  When  not  in  use,  the  coelostat 
and  second  mirror  are  covered  by  a  house  on  wheels,  closed 
at  both  ends  by  walls  of  heavy  canvas.  These  may  be  opened, 
so  that  when  the  house  is  moved  to  the  north  the  coelostat 
stands  fully  exposed.  The  movable  shelter  then  fits  closely 
against  the  south  wall  of  the  laboratory,  and  forms  a  part  of 
the  tube  through  which  the  beam  passes. 


THE  SNOW  TELESCOPE  137 

In  the  preliminary  tests  of  the  Snow  telescope  at  the 
Yerkes  Observatory  the  results  were  rather  disappointing, 
though  good  images  were  sometimes  obtained.  There  was 
evidence  of  distortion  of  the  mirrors  by  the  Sun's  heat,  and 
in  the  first  experiments  on  Mount  Wilson  similar  difficulty 
was  experienced.  Soon  after  the  exposure  of  the  mirrors  to 
the  Sun  it  was  seen  that  the  focal  length  was  increasing,  and, 
as  the  focus  changed,  evidence  of  astigmatism,  due  to  the 
distortion  of  the  plane  mirrors,  made  itself  apparent  in  the 
appearance  of  the  image  inside  and  outside  of  the  focal 
plane.  It  was  soon  found  that  the  focus  changed  much  more 
rapidly  after  the  mirrors  had  been  silvered  for  some  time, 
because  of  the  greater  absorption  of  heat  by  the  slightly  tar- 
nished surfaces.  Moreover,  the  change  was  less  on  a  day 
with  a  cool  breeze  than  on  a  day  with  no  wind.  .  The  question 
then  arose  whether  this  difficulty  could  be  remedied. 

In  the  early  morning,  when,  as  before  stated,  the  defini- 
tion of  the  Sun  is  best,  the  heating  is  much  less  marked  than 
later  in  the  day.  If  the  mirrors  are  shielded  from  sunlight 
between  the  exposures  of  photographs,  and  if  the  exposures 
are  made  as  short  as  possible,  excellent  results  can  be 
obtained  at  this  time,  and  in  the  late  afternoon,  not  long  be- 
fore sunset.  It  has  been  found  advantageous  to  direct  a 
strong  blast  of  air  on  the  surfaces  of  the  mirrors,  by  means 
of  electric  fans,  during  the  exposures  of  the  photographs  and 
the  intervals  between  them. 

It  must  be  understood  that  the  precautions  mentioned  are 
necessary  only  when  it  is  desired  to  secure  the  finest  possible 
definition  of  the  solar  image.  When  such  precautions  are 
used,  the  average  photographs  taken  during  the  summer  in 
the  early  morning  with  the  Snow  telescope  and  a  temporary 
spectroheliograph  are  but  little  inferior  to  the  best  photo- 
graphs, secured  on  only  a  few  days  in  the  year,  with  the  40- 
inch  Yerkes  telescope  and  the  Rumford  spectroheliograph. 


138  STELLAK  EVOLUTION 

The  best  photographs  taken  on  Mount  Wilson  are  distinctly 
superior  to  the  best  secured  in  our  work  with  the  Rum  ford 
spectroheliograph.  It  must  not  be  supposed  that  no  work 
can  be  done  with  the  Snow  telescope  except  under  the  con- 
ditions stated.  As  a  matter  of  fact,  very  fair  photographs 
can  be  obtained  with  the  spectroheliograph  at  almost  any 
time  during  a  cool  day,  and  in  the  early  morning  and  late 
afternoon  hours  of  a  hot  day  without  wind.  It  is  only 
necessary  to  arrange  the  daily  programme  of  observations  so 
that  the  spectroheliograph,  which  requires  the  finest  defini- 
tion, is  used  during  the  period  when  the  seeing  is  best. 
Photographic  work  on  the  spectra  of  sun-spots  follows,  and 
after  this  is  completed  the  conditions  are  entirely  satisfactory 
for  various  other  observations,  such  as  bolographic  work  on 
the  absorption  of  the  solar  atmosphere,  etc.  Some  of  the 
results  obtained  with  the  Snow  telescope  will  be  illustrated 
in  subsequent  chapters. 

From  laboratory  tests,  it  appears  that  the  distortion  of 
mirrors  in  sunlight  is  chiefly  due  to  actual  bending  of  the 
glass,  the  front  surface,  expanded  by  the  heat,  becoming 
convex  and  the  rear  surface  concave.  Radiation  from  an 
electric  heating  coil,  placed  a  short  distance  behind  a  mirror, 
restores  its  figure,  but  not  perfectly.  A  much  better  way  of 
heating  the  back  of  a  mirror  is  by  reflecting  sunlight  upon 
it.  Perhaps  the  best  plan,  however,  is  merely  to  increase 
the  thickness  of  the  glass  mirrors  (p.  235). 


CHAPTER  XVI 

SOME  USES  OP  SPECTROHELIOGRAPH  PLATES 

THE  necessity  of  designing  the  Rumford  spectroheliograph 
for  use  as  an  attachment  of  the  Yerkes  telescope  interfered 
somewhat  with  its  efficiency.  Under  good  conditions  it  gives 
excellent  results,  but  the  limitations  of  aperture,  and  the 
difficulty  of  securing  perfect  equality  in  motion  of  plate  and 
solar  image,  are  sometimes  apparent  in  the  photographs  ob- 
tained with  it.  Fortunately,  the  case  was  different  with  the 
Snow  telescope.  It  was  possible  here  to  adopt  the  most 
satisfactory  form  of  spectroheliograph,  in  which  the  instru- 
ment is  moved  as  a  whole,  while  the  image  of  the  Sun  and 
the  photographic  plate  are  stationary.  The  first  spectrohelio- 
graph of  this  type  was  constructed  in  1893  and  employed 
in  attempts  to  photograph  the  solar  corona  without  an 
eclipse,  from  the  summit  of  Mount  Etna.  For  all  instru- 
ments of  moderate  dimensions,  motion  of  the  spectrohelio- 
graph as  a  whole  appears  to  be  preferable  to  any  mechanical 
contrivance  for  moving  the  plate  and  solar  image  in  syn- 
chronism. 

A  photograph  of  the  spectroheliograph,  mounted  for  use 
with  the  Snow  telescope,  is  reproduced  in  Plate  LXI.  A  better 
idea  of  the  general  design  may  be  obtained  from  Plate  LXII, 
which  shows  the  spectroheliograph  in  our  instrument  shop 
before  it  was  completed.  It  consists  essentially  of  a  massive 
cast-iron  base,  bearing  four  short  V-rails  at  its  four  corners,  on 
which  the  moving  part  of  the  instrument  is  carried  by  four 
steel  balls.  The  cast-iron  platform  which  bears  the  slits  and 
optical  parts  has  four  inverted  A-r&ils>  which  rest  on  the 
steel  balls,  but  almost  its  entire  weight  is  supported  by 

139 


140  STELLAR  EVOLUTION 

mercury,  in  three  tanks  formed  by  subdivisions  in  the  base 
casting.  Wooden  floats  extend  from  the  lower  surface  of 
the  iron  platform  into  these  tanks,  reducing  to  a  minimum 
the  amount  of  mercury  (about  560  pounds)  required  to  up- 
hold the  instrument.  The  motion  of  this  platform  with  re- 
spect to  the  fixed  solar  image  and  photographic  plate  is  pro- 
duced by  either  one  of  two  screws  of  different  pitch,  driven 
by  an  electric  motor  arranged  to  give  a  perfectly  uniform 
speed. 

The  collimator  slit,  on  which  the  solar  image  is  formed, 
is  shown  on  the  right  of  Plate  LXI.  On  account  of  the  large 
size  of  the  solar  image,  which  is  about  6 . 7  inches  in  diameter, 
the  slit  is  8^  inches  long.  After  passing  through  the  slit 
the  light  falls  upon  a  large  collimating  lens  8  inches  in  diam- 
eter, which  renders  the  rays  parallel.  They  then  meet  a 
silvered  glass  mirror,  from  which  they  are  reflected  to  the  two 
prisms,  of  63J°  angle.  After  being  dispersed  by  the  prisms 
the  rays  strike  the  8 -inch  camera  lens,  which  forms  an  image 
of  the  spectrum  on  the  camera  slit  (shown  near  the  center  of 
Plate  LXI).  The  optical  train  thus  resembles  that  of  the 
Rumford  spectroheliograph,  but  the  lenses  and  prisms  are 
so  much  larger  that  no  light  is  lost  from  the  circumference 
of  the  solar  image. 

On  account  of  the  great  curvature  of  the  spectral  lines 
produced  by  such  prisms,  it  would  be  necessary  to  employ  a 
highly  curved  camera  slit,  in  case  an  ordinary  straight  slit 
were  used  to  admit  the  light  from  the  Sun.  In  this  event 
the  resulting  photograph  would  be  greatly  distorted,  because 
points  lying  along  a  straight  line  on  the  Sun  would  appear 
along  a  curved  line  in  the  photographs.  Thus  the  image, 
instead  of  being  circular,  would  be  shaped  somewhat  like 
an  apple,  greatly  flattened  on  one  side.  By  dividing  the 
curvature  evenly  between  the  two  slits  the  distortion  is 
eliminated  and  the  photograph  is  made  circular. 


USES  OF  SPECTROHELIOGKAPH  PLATES        141 

The  actual  operations  in  making  a  photograph  of  the 
flocculi  with  one  of  the  calcium  lines  are  as  follows:  An 
electric  arc,  the  carbons  of  which  have  been  moistened  with 
a  solution  of  calcium  chloride,  is  mounted  in  front  of  the 
collimator  slit.  The  bright  H  and  K  lines  are  easily  visible 
in  the  spectrum  of  such  an  arc,  although  the  same  region  of 
the  solar  spectrum  is  difficult  to  see  distinctly.  By  means  of 
a  micrometer  screw,  the  camera  slit  is  made  to  coincide  with 
one  of  the  lines.  Thus  the  only  light  which  can  reach  the 
photographic  plate  is  that  of  calcium  vapor.  Up  to  this  time 
the  mirror  of  the  coelostat  has  been  shielded  from  the  Sun 
by  a  canvas  screen,  in  order  to  protect  it  from  distortion. 
After  the  photographic  plate  has  been  placed  in  position  in 
its  support  in  front  of  the  camera  slit,  the  canvas  screen  is 
removed  and  the  solar  image  brought  to  a  sharp  focus  on  the 
collimator  slit,  by  moving  the  concave  mirror  of  the  Snow 
telescope  The  slide  of  the  plate-holder  is  then  drawn  and 
the  electric,  motor  started.  The  screw,  driven  by  the  electric 
motor,  then  causes  the  entire  spectroheliograph  to  move  at  a 
slow  and  uniform  rate,  so  that  the  collimator  slit  passes  over 
the  solar  image  and  the  camera  slit  moves  across  the  photo- 
graphic plate. 

If  it  is  desired  to  take  a  photograph  with  a  hydrogen  line, 
instead  of  a  calcium  line,  the  prisms  and  mirror  are  adjusted 
until  the  line  in  question  falls  upon  the  camera  slit,  when 
the  exposure  is  made  as  before. 

In  the  daily  programme  of  observations  at  least  one  photo- 
graph with  the  Hj  line  of  calcium,  showing  the  faculae  and 
low  level  calcium  vapor;  one  with  the  H2  line  of  calcium, 
showing  the  flocculi  at  a  higher  level ;  one  with  the  Hy  line 
of  hydrogen;  and  one  with  an  iron  line,  are  made  in  the 
early  morning  and  again,  if  circumstances  permit,  in  the  late 
afternoon  (Plates  LXIII-LXVII).  Since  the  weather  is 
clear  day  after  day  through  the  summer  and  autumn  months 


142  STELLAK  EVOLUTION 

(on  112  consecutive  days  in  the  summer  of  1907),  and  not 
infrequently  during  the  rainy  season,  the  instrument  thus 
yields  a  large  number  of  plates,  suitable  for  the  comparative 
study  of  the  flocculi. 

Photographs  of  the  prominences  are  also  made  daily,  when 
circumstances  permit.  These  are  used  to  determine  the 
changes  in  number  and  total  area  of  the  prominences  during 
the  Sun-spot  period. 

In  the  establishment  of  an  observatory  much  remains  to 
be  done  after  successful  photographs  of  astronomical  phe- 
nomena have  been  obtained.  Indeed,  although  the  work  of 
organization  must  be  far  advanced  before  photographs  can 
be  secured,  the  most  important  steps  are  still  to  be  taken. 
For  an  astronomical  photograph,  while  it  may  yield  much 
new  information  from  casual  examination,  is  to  be  regarded 
as  a  document  of  great  value,  worthy  of  prolonged  investi- 
gation. Every  photograph  of  the  Sun,  for  example,  repre- 
sents its  changing  phenomena  as  they  were  at  the  moment 
of  the  exposure,  under  conditions  which  will  never  be  exactly 
repeated.  The  best  methods  of  obtaining  from  photographs 
all  the  knowledge  they  are  capable  of  conveying  are  to  be 
arrived  at  only  after  the  fullest  consideration  of  the  possi- 
bilities. 

In  chap,  xi  the  most  striking  characteristics  of  the  floc- 
culi have  been  explained  and  illustrated.  We  must  now 
consider  how  these  objects  may  be  systematically  studied,  in 
such  a  way  as  to  contribute  to  our  knowledge  of  the  solar 
constitution.  The  most  obvious  peculiarity  of  the  flocculi, 
apart  from  their  change  in  form,  is  their  motion  across  the 
Sun's  disk.  This  is  due  to  the  solar  rotation,  which  was 
first  discovered  through  the  daily  motion  of  sun-spots.  It 
is  remarkable  that  the  spots  do  not  move  as  they  would  if 
they  were  fixed  to  the  surface  of  a  solid  sphere.  Spots  in 
different  latitudes  move  with  different  angular  velocities,  and 


USES  or  SPECTROHELIOGRAPH  PLATES        143 

exhibit  what  is  called  the  "equatorial  acceleration;"  i.  e., 
spots  near  the  Sun's  equator  complete  a  revolution  in  much 
shorter  time  than  those  in  higher  latitudes.  At  the  equator 
the  rotation  period  is  about  twenty-five  days.  At  10°  north 
or  south  latitude  the  period  is  several  hours  longer,  and  at 
45°  it  is  about  twenty-seven  and  a  half  days.  The  faculae, 
according  to  results  obtained  by  Stratonoff  and  others,  follow 
the  same  general  law.  Spectroscopic  observations,  based  on 
an  application  of  Doppler's  principle  show  that  the  motion 
is  not  confined  to  the  spots  and  faculae,  but  is  also  shared 
by  the  layer  of  metallic  vapors  (the  "reversing  layer")  which 
lies  just  above  the  photosphere,  and  produces  the  dark  lines 
of  the  solar  spectrum  by  absorption  of  the  white  light 
coming  through  it  from  below.  It  thus  becomes  interesting 
to  inquire  whether  the  calcium  flocculi,  which  we  suppose  to 
be  clouds  of  luminous  vapor  lying  at  an  elevation  of  several 
thousand  miles  above  the  photosphere,  show  a  similar  law 
of  rotation. 

The  method  employed  to  determine  the  rotation  period  of 
the  spots  is  to  measure  their  latitude  and  longitude,  referred 
to  the  center  of  the  Sun,  on  plates  taken  at  intervals  of  one 
or  more  days,  and  in  this  way  to  ascertain  the  change  of 
longitude  of  the  same  spot  in  twenty-four  hours.  By  thus 
obtaining  the  velocities  -of  spots  in  different  latitudes  the  law 
of  rotation  can  be  derived.  In  considering  the  methods  of 
measuring  the  latitude  and  longitude  of  a  spot,  we  must 
remember  that  the  plane  of  the  Sun's  rotation  is  inclined  at 
an  angle  of  about  7°  with  that  of  the  Earth's  orbit.  The 
Earth  passes  through  the  nodes  (the  intersection  of  this 
plane  with  the  ecliptic)  about  June  3  and  December  5,  and 
only  on  these  dates  do  the  spots  appear  to  move  in  straight 
lines  across  the  disk.  The  angle  between  the  Sun's  axis  and 
the  north  and  south  line  in  the  sky  (called  the  "position 
angle"  of  the  Sun's  axis)  varies  about  53°  in  the  course  of 


144  STELLAE  EVOLUTION 

the  year — about  26 J°  each  side  of  zero.  It  is  thus  evident 
that  in  determining  the  latitude  and  longitude  of  a  spot  by 
ordinary  methods  of  measurement  considerable  calculation 
will  be  required.  The  process  employed  at  Greenwich,  on 
the  direct  photographs  of  the  Sun  obtained  there,  is  to  meas- 
ure the  distance  of  the  spot  from  the  center,  and  the  angle 
between  the  Sun's  axis  and  the  line  joining  the  spot  with 
the  center  of  the  disk.  As  the  inclination  of  the  Sun's  axis 
is  known  for  every  day  in  the  year,  it  then  becomes  possible 
to  calculate  the  latitude  and  longitude  of  the  spot. 

This  method  is  very  satisfactory  when  a  comparatively 
small  number  of  objects  are  to  be  measured  on  each  plate, 
which  is  the  case  with  sun-spots.  But  the  flocculi  are  so 
numerous,  and  offer  so  many  points  suitable  for  measure- 
ment, that  the  calculations  required  for  each  spectrohelio- 
graph  plate  would  be  very  extensive.  In  seeking  to  find 
some  simple  method  of  abridging  these  calculations,  it 
appeared  that  the  solar  photograph  might  be  projected 
upon  the  surface  of  a  globe  ruled  with  meridians  and 
parallels  1°  apart.  The  axis  of  the  globe  being  set  at  the 
inclination  corresponding  to  the  date  of  the  photograph,  it 
should  then  be  possible  to  read  off  the  latitude  and  longitude 
directly,  by  estimating  the  position,  in  tenths  of  a  degree, 
of  the  flocculus  in  question,  with  reference  to  the  nearest 
meridian  and  parallel  (Plate  LXIX).  As  the  longitude  of 
the  center  of  the  Sun's  disk  is  tabulated  for  each  day  in  the 
year,  no  calculations  would  be  necessary,  except  to  add  or 
subtract  this  longitude  in  the  case  of  each  of  the  readings. 

This  method  proved  so  satisfactory,  when  used  at  the 
Yerkes  Observatory  in  measuring  the  Kenwood  photographs, 
that  it  was  afterward  adopted,  in  perfected  form,  in  the 
Computing  Division  of  the  Solar  Observatory.  The  new 
globe-measuring  machine,  or  "heliomicrometer,"  is  illus- 
trated in  Plate  LXX.  Two  4-inch  telescopes,  shown  in  the 


USES  OF  SPECTROHELIOGRAPH  PLATES        145 

upper  part  of  the  cut,  are  pointed  toward  two  plane  silvered 
glass  mirrors  thirty  feet  away.  One  of  these  mirrors  receives 
light  from  the  spectroheliograph  plate,  which  is  mounted 
immediately  tinder  the  right-hand  telescope  and  illuminated 
by  incandescent  lamps  from  behind.  The  other  receives 
light  from  a  globe,  mounted  below  the  left-hand  telescope 
and  illuminated  on  its  front  surface.  The  images  of  globe 
and  plate,  given  by  the  two  telescopes,  are  brought  together 
in  a  single  eye-piece,  so  that  the  observer  sees  them  super- 
posed. If,  then,  the  surface  of  the  globe  is  ruled  with 
meridians  and  parallels,  as  in  the  instrument  previously 
described,  the  positions  of  the  flocculi  can  be  read  off  by 
estimation.  However,  it  is  desired  in  this  case  to  attain  a 
higher  degree  of  precision  in  the  measurements,  and  to  see 
small  and  faint  flocculi  to  better  advantage  than  would  be 
possible  if  they  were  observed  in  projection  against  the  illu- 
minated surface  of  the  globe.  Accordingly,  a  pair  of  cross- 
hairs, which  can  be  moved  over  the  plate  in  a  horizontal  or 
vertical  direction  by  the  observer  at  the  eye-piece,  is  made 
to  coincide  with  the  object  to  be  measured.  The  globe  is 
then  illuminated,  and  rotated  in  latitude  and  longitude  until 
a  point  corresponding  to  the  intersection  of  the  equator  and 
the  central  meridian  falls  exactly  upon  the  cross-hairs.  A 
circle,  which  can  be  read  by  the  observer  at  the  eye-piece, 
then  shows  the  angle  through  which  the  globe  has  been 
turned  in  latitude.  A  second  circle  gives  the  distance  in 
longitude  from  the  center  of  the  Sun.  It  is,  of  course,  to  be 
understood  that  the  axis  about  which  the  globe  is  turned  in 
measuring  longitudes  is  set  at  the  proper  inclination  for  the 
date  of  the  photograph.  For  less  precise  measurements,  the 
position  of  the  cross-hairs  may  be  estimated  with  reference 
to  the  rulings  on  the  (fixed)  globe. 

This  instrument,  which  was  constructed  in  the  shop  of 
the  Solar  Observatory,  has  proved  very  satisfactory  in  prac- 


146  STELLAR  EVOLUTION 

tice.  It  has  been  found  that  the  latitudes  and  longitudes, 
thus  read  off  directly,  are  as  accurate  as  when  determined 
by  measuring  the  plate  in  an  ordinary  measuring-machine 
and  performing  the  necessary  calculations.  Since  the  meas- 
urements can  be  be  made  quite  as  rapidly  on  the  heliomi- 
crometer  as  on  the  other  machine,  all  the  time  required  to 
make  the  calculations  is  saved.  Thus  one  observer  can 
measure  a  great  number  of  flocculi,  and  the  services  of  sev- 
eral computers  are  rendered  unnecessary. 

A  discussion  of  the  measurements  made  in  this  way  shows 
that  the  flocculi  follow  a  law  of  rotation  similar  to  that  which 
governs  the  spots  and  faculae.  It  will  require  some  time  to 
learn  whether  the  velocities  of  the  flocculi  differ  appreciably 
from  those  of  the  spots.  It  appears  probable,  however,  from 
results  thus  far  obtained,  that  the  flocculi  move  with  about 
the  same  velocity  as  the  faculae.  This  would  be  a  natural 
result,  since,  as  already  explained,  the  vapors  of  the  flocculi 
probably  rise  from  the  faculae,  and  lie  immediately  above 
them. 

The  importance  of  providing  for  the  closest  possible  cor- 
relation between  all  of  the  investigations  of  the  Solar  Obser- 
vatory has  already  been  mentioned.  For  this  reason  studies 
of  the  solar  rotation  should  be  made  with  reference  to  other 
solar  work.  The  motions  of  individual  flocculi  frequently 
differ  considerably  from  the  average  motions  of  the  flocculi  in 
the  same  latitude.  Such  differences,  in  many  instances,  are 
doubtless  similar  to  those  observed  in  the  case  of  sun-spots, 
where  they  are  related  to  the  spot's  activity,  which  varies 
greatly  during  the  course  of  its  development.  However,  the 
daily  motion  of  a  flocculus  may  also  depend  upon  its  height 
above  the  photosphere,  and  this  may  vary  from  day  to  day. 
It  thus  becomes  desirable  to  learn  whether  differences  in  the 
height  of  flocculi  can  be  detected  and  actually  measured. 
For  example,  do  the  hydrogen  flocculi  lie  at  an  average  level 


USES  OF  SPECTROHELIOGRAPH  PLATES        147 

in  the  solar  atmosphere  above  or  below  that  of  the  calcium 
flocculi,  and,  if  so,  do  they  show  differences  of  rotational 
velocity  that  may  depend  upon  this  fact? 

It  has  already  been  explained,  in  chap,  xi,  that  the  cal- 
cium flocculi  photographed  when  the  bright  H2  or  K2  line 
is  employed  probably  lie  above  the  bright  objects  of  similar 
form,  but  somewhat  smaller  area,  which  are  photographed 
when  the  slit  is  set  on  the  broad  Hj  or  Kj  band.  It  is  not  so 
obvious,  however,  that  the  average  level  of  the  hydrogen  floc- 
culi is  above  that  of  the  H2  and  K2  calcium  flocculi,  but  this  can 
be  determined  by  accurate  measurements.  The  forms  of  the 
dark  hydrogen  flocculi,  as  already  remarked,  closely  resemble 
those  of  the  bright  calcium  flocculi,  though  in  many  cases 
there  are  important  differences  (Plates  LXXI  and  LXXII).1 
With  the  aid  of  the  stereocomparator,  an  instrument  manu- 
factured by  the  Zeiss  Optical  Company  for  the  purpose  of 
making  accurate  comparisons  of  photographs,  it  is  possible 
to  observe  a  hydrogen  photograph  in  superposition  upon  a 
calcium  photograph,  taken  within  so  short  an  interval  of  time 
that  no  appreciable  change  occurred  on  the  Sun  between  the 
exposures.  With  the  monocular  eye-piece  of  the  instrument 
the  two  photographs,  in  precise  superposition,  are  observed  in 
quick  succession.  For  this  purpose  a  device  is  used  which 
permits  the  eye  to  see  one  of  the  plates,  and,  immediately 
afterward,  the  other.  If  a  micrometer  wire  is  set  on  a  cal- 
cium flocculus  lying  near  the  edge  of  the  Sun,  and  the  image 
of  the  corresponding  hydrogen  flocculus  is  then  brought  into 
view,  it  is  found  to  be  displaced  slightly  away  from  the  center 
of  the  disk.  This  is  not  true  of  all  the  hydrogen  flocculi. 
On  the  average,  however,  these  dark  hydrogen  clouds  seem  to 

1  These  photographs  were  separated  by  an  interval  of  2h  26m,  during  which  time 
the  changes  in  the  forms  of  the  flocculi  would  not  ordinarily  be  sufficiently  marked 
to  interfere  with  the  general  comparison  of  the  more  conspicuous  features.  In  this 
case,  however,  the  changes  may  have  been  rapid,  since  the  numerous  bright  flocculi 
near  the  spot  indicate  great  eruptive  activity.  For  the  accurate  comparison  of 
details,  the  photographs  must  be  taken  simultaneously. 


148  STELLAR  EVOLUTION 

be  displaced  in  this  way,  by  an  amount  representing  a  height 
of  some  1,500  miles  above  the  corresponding  calcium  clouds.1 
It  will  therefore  be  interesting  to  determine  at  some  future 
time  whether  the  rotational  velocity  of  the  hydrogen  flocculi 
differs  appreciably  from  that  of  the  calcium  flocculi. 

An  important  step  in  the  interpretation  of  spectrohelio- 
graph  plates  will  be  made  when  it  can  be  ascertained  whether 
anomalous  dispersion  plays  any  part  in  producing  the  phe- 
nomena recorded  by  them.  Our  present  views  as  to  the 
nature  of  Sun-spots,  prominences,  and  other  solar  phenomena 
are  based  on  the  assumption  that  their  light  reaches  us  along 
nearly  straight  lines.  If  the  pressure  in  the  region  through 
which  the  rays  pass  is  low,  this  may  be  essentially  true  for 
white  light.  But  we  know  that  light  of  about  the  same 
wave-length  as  that  of  an  absorption  line  in  the  spectrum,  is 
bent  far  out  of  a  straight  path  when  it  passes  through  the 
vapor  to  whose  absorption  the  line  is  due.  The  conse- 
quences of  this  fact  have  led  Julius  to  develop  a  new  solar 
theory,  based  on  the  supposition  that  all  metallic  vapors  at 
any  given  distance  from  the  Sun's  center  are  completely 
mixed,  but  not  of  uniform  density  throughout.  Under  these 
circumstances  the  chromosphere,  prominences,  and  flocculi 
would  not  exist  as  we  see  them,  but  such  appearances  might 
be  caused  by  anomalous  dispersion  of  light  passing  out 
through  the  vapors  from  the  interior  of  the  Sun.  A  series 
of  investigations,  involving  solar,  stellar,  and  laboratory  work, 
is  being  carried  out  on  Mount  Wilson  for  the  purpose  of 
testing  this  theory. 

The  rotation  periods  of  sun-spots  may  depend  upon  their 
level,  and  this  raises  the  old  question  as  to  the  position  of 
these  objects  with  respect  to  the  photosphere.  According 
to  the  common  view  sun-spots  are  saucer-shaped  cavities  in 
the  photosphere.  This  idea  is  based  upon  the  observations 

i  This  result  must  be  checked  on  photographs  taken  simultaneously. 


USES  OF  SPECTROHELIOGRAPH  PLATES   149 

of  Wilson,  who  found  that  when  a  spot  is  carried  toward  the 
limb  by  the  solar  rotation,  the  penumbra,  on  the  side  toward 
the  center  of  the  disk,  is  reduced  in  apparent  width,  as  it 
would  be,  on  account  of  its  inclination  to  the  line  of  vision,  if 
it  sloped  downward  toward  the  umbra.  The  best  modern 
results  do  not  offer  any  certain  confirmation  of  this  view,  and 
thus  render  necessary  an  appeal  to  some  independent  test  of 
the  question.  Ten  years  ago  it  was  pointed  out  by  Frost 
that  the  heat  radiation  of  a  spot,  as  compared  with  that  of  the 
neighboring  photosphere,  increases  as  the  spot  approaches 
the  limb.  From  this  it  was  naturally  concluded  that  the  spot 
must  lie  above  the  photosphere,  at  such  a  level  as  to  escape 
the  influence  of  the  low-lying  absorbing  veil,  which  so  greatly 
reduces  the  intensity  of  the  photospheric  light  at  the  solar 
circumference.  It  has  recently  been  found,  however,  that 
sun-spots  radiate  a  much  smaller  proportion  of  violet  light 
than  the  photosphere.  As  violet  light  is  always  reduced  by 
an  absorbing  atmosphere  in  much  larger  proportion  than 
light  of  longer  wave-length,  it  follows  that  the  observed 
effect  would  be  seen  in  the  case  of  sun-spots,  even  if  they 
were  at  the  same  level  as  the  photosphere.  To  remove 
the  difficulty  it  is  only  necessary  to  confine  the  comparative 
measures  to  a  single  color,  rather  than  to  use  the  total  radia- 
tion, comprising  light  of  all  wave-lengths. 

The  spectroheliograph  affords  a  simple  means  of  accom- 
plishing this.  It  is  employed  to  make  photographs  of  a  sun- 
spot  and  the  surrounding  photosphere  on  various  dates,  corre- 
sponding to  the  changing  position  of  the  spot  on  the  solar 
disk.  In  making  these  photographs  the  camera  slit  is  set,  not 
on  any  of  the  spectral  lines,  but  on  a  space  between  the  lines, 
preferably  in  the  yellow  or  red,  since  the  influence  of  extra- 
neous light  will  be  least  marked  in  this  region.  On  account 
of  the  darkness  of  the  spot,  which  would  require  an  exposure 
about  six  times  as  long  as  that  for  the  photosphere  to  give  a 


150  STELLAE  EVOLUTION 

photograph  of  equal  intensity,  it  is  desirable  to  decrease  the 
intensity  of  the  photospheric  light  by  a  dark  glass,  placed  over 
the  slit,  but  so  arranged  as  not  to  reduce  the  light  from  the 
sun-spot.  In  this  way  the  spot  and  photospheric  light  can  be 
compared  from  day  to  day,  by  means  of  photometric  meas- 
urements. The  same  method  can  be  employed  to  measure 
the  level  of  the  flocculi.  Such  work  is  now  in  progress  at  the 
Solar  Observatory,  in  conjunction  with  the  other  investiga- 
tions already  mentioned.  Since  the  level  of  a  spot  may 
affect  its  temperature,  and  therefore  its  spectrum,  an  attempt 
will  be  made  to  correlate  this  work,  not  only  with  determina- 
tions of  the  spot's  motion,  but  also  with  the  spectroscopic 
observations  described  in  chap.  xvii. 

These  few  examples  may  suffice  to  give  an  idea  of  the 
character  of  the  work  done  with  the  spectroheliograph  of  the 
Snow  telescope.  The  vertical  motion  of  the  calcium  vapor 
in  the  flocculi;  the  manner  in  which  it  flows  horizontally 
over  sun-spots ;  the  relationship,  in  point  of  development,  of 
flocculi  to  spots;  and  other  similar  matters,  are  also  studied 
systematically.  It  may  also  be  added  that  the  area  of  the 
flocculi  is  measured  on  each  day's  plates,  since  it  serves  as  an 
index  to  the  Sun's  activity,  which  may  prove  important  when 
considered  in  its  bearing  on  possible  variations  of  the  solar 
radiation  and  their  effect  on  terrestrial  phenomena. 


CHAPTEK  XVII 
A  STUDY  OF  SUN-SPOTS 

IT  has  already  been  remarked  (p.  69)  that  sun-spots, 
though  apparently  much  darker  than  the  photosphere,  are,  in 
reality,  brilliantly  luminous  objects.  Though  they  thus  ap- 
pear dark  merely  by  contrast,  the  cause  of  their  reduced 
brilliancy  has  given  rise  to  much  discussion.  Some  of  the 
most  recent  theories  have  maintained  that  sun-spots  are  so 
much  hotter  than  other  parts  of  the  solar  surface  that  the 
photospheric  clouds,  due  to  condensation  of  the  vapors  rising 
from  the  Sun's  interior,  cannot  form  at  these  points.  One  of 
Lockyer's  arguments  in  support  of  his  hypothesis  that  the 
terrestrial  elements  are  dissociated  at  the  high  temperature 
of  the  stars  is  based  upon  the  view  that  at  times  of  sun-spot 
maxima  the  spots  are  too  hot  to  permit  certain  of  the  terres- 
trial elements  to  exist  in  them.  This  conclusion  was  founded 
upon  a  long  series  of  observations  of  certain  lines  in  the 
spectra  of  sun-spots.  The  spot  spectrum  differs  from  the 
solar  spectrum  in  the  fact  that  some  of  the  solar  lines  are 
strengthened  or  widened,  some  are  weakened,  and  many  are 
unchanged  (Plate  LXXIV).  The  number  of  lines  whose 
intensities  are  thus  altered  amounts  to  many  hundreds; 
indeed,  if  the  fainter  lines  are  taken  into  account,  to  several 
thousands.  Lockyer's  observations  consist  in  recording,  on 
every  clear  day,  the  "twelve  most  widened  lines"  in  the 
spectra  of  spots  then  visible  on  the  Sun.  His  results 
seemed  to  indicate  that  at  sun-spot  minima  the  most  widened 
lines  represent  known  substances;  while  at  sun-spot  maxima 
many  of  these  give  place  to  unknown  lines,  which  he  attrib- 
uted to  unknown  substances  produced  by  dissociation  of  the 

151 


152  STELLAR  EVOLUTION 

elements  at  the  high  temperature  assumed  to  be  character- 
istic of  periods  of  greatest  solar  activity.  Some  of  his  later 
papers  favor  the  view  that  sun-spots1  possess  a  lower  tem- 
perature than  would  thus  be  indicated,  and  he  may  therefore 
have  decided  to  abandon  the  conclusions  based  on  his  earlier 
spectroscopic  observations. 

In  spite  of  these  results,  and  of  all  the  theories  which 
attribute  high  temperature  to  sun-spots,  the  more  common 
opinion  has  been  that  they  are  regions  of  reduced  tempera- 
ture. This  view  has  been  based  partly  upon  their  decreased 
brightness,  as  compared  with  the  photosphere,  and  partly 
upon  the  presence  in  their  spectra  of  certain  bands  which, 
though  unidentified,  were  supposed  to  represent  molecules 
that  cannot  exist  at  the  high  temperature  of  the  Sun.  Accu- 
rate knowledge  of  these  bands,  however,  was  almost  entirely 
lacking,  on  account  of  their  faintness  and  the  extreme  diffi- 
culty of  observing  them  visually. 

It  seemed  probable  that  progress  in  this  department  of 
solar  research  might  be  expected  to  result  from  the  success- 
ful application  of  photography  to  the  study  of  spot  spectra, 
Experiments  made  with  this  object  in  view  at  the  Kenwood 
Observatory  showed  some  of  the  principal  widened  lines, 
but  failed  to  give  the  details  needed  for  satisfactory  work. 
These  results  were  surpassed  by  photographs  made  by  Young 
with  the  23-inch  Princeton  refractor,  but  here  also  the  need 
of  more  powerful  instrumental  means  seemed  to  be  apparent. 
The  Kenwood  experiments  were  continued  with  the  40-inch 
Yerkes  telescope,  and  some  of  the  "band  lines,"  first  observed 
visually  by  Maunder,  were  photographed,  in  addition  to  many 
of  the  widened  lines.  However,  there  was  reason  to  believe 
that  much  better  results  could  be  obtained  with  the  aid  of  a 
long-focus  grating  spectrograph,  capable  of  photographing 

1  Because  of  the  great  strength  of  the  titanium  lines  in  Arcturus. 


A  STUDY  OF  SUN-SPOTS  153 

the  spectrum  on  a  large  scale.  Further  work  was  therefore 
deferred  until  it  could  be  taken  up  with  the  Snow  telescope 
and  a  powerful  Littrow  or  auto-collimating  spectrograph. 

This  spectrograph  is  of  a  very  simple  type.  The  image  of 
the  Sun  is  formed  on  a  slit,  s,  through  which  the  light  passes  to 
a  6-inch  collimating  objective,  o,  of  18  feet  focal  length,  which 
renders  the  rays  parallel  (Fig.  6).  The  rays  then  fall  upon 
a  plane  grating,  g,  which  diffracts  them  into  a  series  of 
spectra.  Light  from  a  portion  of  one  of  these  spectra  returns 
to  the  objective,  o,  which  forms  an  image  of  the  spectrum  on 


FIG.  6 
Path  of  Rays  in  Littrow  Spectrograph 

a  photographic  plate,  p,  standing  just  above  the  slit.  In 
order  to  form  the  image  at  this  point  the  grating  must  be 
slightly  inclined  backward,  so  as  to  send  the  beam  upward. 

This  instrument,  as  mounted  for  use  with  the  Snow  tele- 
scope, is  shown  in  Plate  LXXIII.  As  the  tube  of  the  spec- 
trograph stands  immediately  above  the  spectroheliograph,  a 
section  of  it  can  be  rotated  out  of  the  way,  to  permit  access 
to  the  prism-train  of  the  latter  instrument.  When  the 
spectrograph  is  to  be  used  instead  of  the  spectroheliograph, 
the  concave  telescope  mirror  is  moved  north  through  a  suffi- 
cient distance  to  transfer  the  focal  plane  from  the  spectro- 
heliograph slit  to  the  spectrograph  slit.  Then,  by  inclining 
the  mirror  backward  through  a  small  angle,  the  solar  image 
is  raised  to  the  proper  height.  After  final  focusing,  a  sun- 
spot  is  brought  exactly  upon  the  slit  with  the  aid  of  slow- 
motion  electric  motors,  connected  with  the  concave  mirror 
and  controlled  from  a  point  near  the  focal  plane. 

In  photographing  the  spectrum  of  a  sun-spot,  all  light  is 


154  STELLAR  EVOLUTION 

excluded  from  the  spectrograph  except  that  which  comes  from 
the  umbra.  This  is  done  by  covering  all  of  the  slit  except 
a  small  portion  at  the  center.  The  dispersion  of  the  second 
or  third  order  of  the  grating  is  usually  employed.  After  this 
exposure  has  been  completed,  the  center  of  the  slit  is  covered 
and  light  from  the  photosphere  admitted  on  each  side.  This 
gives  a  narrow  photograph  of  the  spot  spectrum  between 
two  strips  of  solar  spectra  (Fig.  1,  Plate  LXXIV). 

Casual  examination  of  the  spot  spectra  thus  recorded  is 
sufficient  to  show  that  the  problem  of  interpreting  them  is 
not  a  simple  one.  If  we  consider,  for  example,  the  lines  of 
some  single  element  represented  in  the  spot,  we  find  that  they 
are  not  all  affected  alike.  Some  are  greatly  strengthened, 
or  perhaps  attended  by  broad,  faint  wings.  The  former  effect 
is  so  very  pronounced,  in  certain  cases,  that  lines  wholly 
invisible  in  the  solar  spectrum  are  among  the  most  conspicu- 
ous of  the  spot  lines.  Some  of  the  solar  lines,  on  the  con- 
trary, are  greatly  weakened,  or  entirely  absent  in  the  spot 
spectrum.  Finally,  there  are  many  spot  lines  of  unchanged 
intensity.  Examples  of  most  of  these  phenomena  are  illus- 
trated in  Plates  LXXIV  and  LXXVI. 

In  order  to  interpret  such  results,  it  is  necessary  to  have 
recourse  to  laboratory  experiments.  It  might  be  supposed 
that  the  required  knowledge  of  terrestrial  spectra  would  be 
available  in  the  literature  of  spectroscopy.  This,  however, 
is  not  the  case.  It  is  true  that  the  lines  in  the  spectra  of 
most  of  the  elements  have  been  measured,  and  many  experi- 
ments have  been  made  on  the  changes  in  spectra  produced 
by  varying  the  conditions  under  which  the  vapors  emit  their 
radiations.  It  usually  happens,  however,  when  one  attempts 
to  apply  published  results  to  the  interpretation  of  solar 
phenomena,  that  the  data  required  for  the  solution  of  the 
particular  problem  in  hand  are  lacking.  Pressure,  for 
example,  is  known  to  displace  spectral  lines  toward  the  red, 


A  STUDY  or  SUN-SPOTS  155 

and  the  actual  shifts  of  certain  lines  of  several  different  ele- 
ments have  been  measured.  But  these  form  a  very  small  per- 
centage of  the  total  number  of  lines  in  the  spectra  of  these 
substances,  and  the  shifts  of  any  lines  that  happen  to  be 
under  investigation  are  rarely  found  in  the  published  tables. 
The  same  may  be  said  of  the  effect  of  temperature  on  spectra. 
It  has  long  been  known  that  a  reduction  of  temperature 
increases  the  relative  brightness  of  certain  lines,  decreases 
that  of  others,  and  is  without  effect  on  the  rest  of  the  spec- 
trum. Indeed,  it  was  even  known  that  some  of  the  iron  lines 
which  are  prominent  at  low  temperatures  are  among  the  more 
conspicuous  lines  of  spot  spectra.  But  these  instances  were 
so  few  and  scattered  that  no  safe  inferences  could  be  based 
upon  them.  Moreover,  it  had  not  been  definitely  proved  that 
these  changes  of  relative  intensity  could  actually  be  produced 
by  temperature  alone.  Most  of  the  experiments  showing 
variations  of  spectra  have  involved  the  use  of  electric  dis- 
charges, where  causes  are  at  work  which  might  have  a  far 
greater  influence  than  temperature  change  on  the  character 
of  the  spectra.  Examples  might  easily  be  multiplied  to  show 
that  the  study  of  solar  and  stellar  physics  cannot  be  carried 
on  effectively  without  a  constant  appeal  to  laboratory  experi- 
ments, planned  with  special  reference  to  the  needs  of  the 
particular  problem  under  investigation. 

For  this  reason  much  stress  has  been  laid  in  the  equip- 
ment of  the  Solar  Observatory  upon  the  provision  of  suitable 
laboratory  facilities.  It  seemed  essential,  in  designing  the 
spectroscopic  laboratory  on  Mount  Wilson,  not  only  to  in- 
clude a  considerable  number  of  light-sources,  which  could 
be  examined  under  various  conditions  of  temperature,  pres- 
sure, etc.,  but  also  to  arrange  them  in  such  a  way  that  the 
appeal  to  one  or  the  other  condition  could  be  made  without 
the  delays  ordinarily  experienced  when  apparatus  must  be 
specially  set  up  for  a  certain  investigation.  In  the  desired 


156  STELLAR  EVOLUTION 

plan  the  apparatus  must  be  always  ready,  needing  only  the 
turning-on  of  an  electric  current,  or  the  adjustment  of  a 
mirror,  to  bring  it  into  action.  It  is  not  so  much  a  question 
of  the  saving  of  time,  which  the  provision  of  these  means 
undoubtedly  offers,  as  it  is  of  the  greatly  increased  efficiency 
of  the  working  programme  thus  rendered  possible.  The 
immediate  imitation  in  the  laboratory,  under  experimental 
conditions  subject  to  easy  trial,  of  solar  and  stellar  phe- 
nomena, not  only  tends  to  clear  up  obscure  points,  but  pre- 
pares the  way  for  the  development  along  logical  lines  of 
the  train  of  reasoning  started  by  the  astronomical  work. 
Questions  are  constantly  arising  which,  if  partially  or  wholly 
answered  by  suitable  laboratory  experiments,  may  modify 
in  an  important  way  the  daily  programme  of  astronomical 
observations. 

The  arrangement  of  the  apparatus  in  the  spectroscopic 
laboratory  of  the  Yerkes  Observatory  has  already  been 
described  (p.  107).  At  the  Solar  Observatory  an  improved 
plan  has  been  adopted.  Instead  of  a  circular  wooden  table, 
an  annular  concrete  pier  is  employed,  giving  space  on  the 
inner  wall  for  the  various  switches  used  to  control  the  cur- 
rent supplied  to  the  different  sources,  and  also  permitting 
the  observer  to  inspect  any  light-source  from  the  direction 
of  the  plane  mirror  at  the  center  of  the  pier.  Instead  of  a 
single  plane  mirror,  two  are  provided,  capable  of  rotating 
independently  of  one  another  about  the  same  vertical  axis. 
When  the  Littrow  spectrograph  is  used  to  photograph  the 
spectrum  of  any  of  the  light-sources,  only  the  lower  plane 
mirror  is  in  action.  By  setting  this  at  the  proper  angle, 
light  from  any  source  on  the  annular  pier  can  be  sent  to 
a  concave  mirror  (seen  near  the  middle  of  Plate  LXXV), 
which  forms  an  image  on  the  slit  of  the  Littrow  spectro- 
graph. If  low  dispersion,  rather  than  high  dispersion,  is 
required,  a  one-prism  quartz  spectrograph  is  used.  Again, 


A  STUDY  OF  SUN-SPOTS  157 

for  the  special  study  of  certain  lines  under  the  highest  resolv- 
ing power,  particularly  in  investigations  of  the  Zeeman  effect, 
an  echelon  spectroscope  is  used.  In  either  case  the  concave 
mirror  is  tipped  back  at  a  small  angle,  so  as  to  return  the 
light  to  the  upper  plane  mirror,  from  which  it  is  reflected  to 
the  slit  of  one  of  these  instruments.  In  Plate  LXXV  the 
quartz  spectrograph  may  be  seen  just  above  the  concave 
mirror,  while  the  echelon  spectroscope  stands  on  the  extreme 
right,  near  the  end  of  the  room.  The  Littrow  spectrograph, 
which  is  ordinarily  employed,  is  similar  in  type  to  the  spec- 
trograph used  with  the  Snow  telescope.  The  rectangular  box 
which  carries  the  slit  and  plate-holder  of  this  instrument  is 
shown  on  the  pier  in  the  lower  left  corner  of  Plate  LXXV. 
The  following  apparatus  stands  on  the  annular  pier:  the 
first  instrument  on  the  right  is  a  powerful  electro-magnet, 
used  for  the  study  of  the  Zeeman  effect — i.  e.,  the  influence 
of  a  magnetic  field  in  separating  spectral  lines  into  several 
components.  For  example,  in  the  spectrum  of  a  spark  passing 
between  iron  terminals  most  of  the  lines  appear  single,  even 
when  observed  with  the  great  resolving  power  of  an  echelon 
spectroscope.  If,  however,  the  spark  is  placed  between  the 
poles  of  a  powerful  magnet,  the  effect  of  the  magnetic  field 
is  to  break  each  line  up  into  several  components.  It  would 
take  us  too  far  away  from  our  immediate  subject  to  discuss 
the  theoretical  questions  which  underlie  these  phenomena. 
It  may  be  said,  however,  that  by  observing  whether  certain 
lines  behave  similarly  under  the  influence  of  a  magnetic 
field,  we  can  tell  whether  they  would  be  expected  to  act 
together  in  the  Sun.  It  is  not  a  question  here  of  detecting 
magnetic  phenomena  in  the  Sun,  since  most  careful  study 
has  not  revealed  any  evidence  of  solar  magnetic  fields  capable 
of  affecting  the  appearance  of  the  spectral  lines.  Neverthe- 
less, the  method  provides  an  arbitrary  means  of  picking  out 
certain  groups  of  lines,  which  may  be  so  intimately  related 


158  STELLAR  EVOLUTION 

to  one  another  that  we  should  expect  them  always  to  behave 
alike  when  observed  in  the  Sun  or  stars. 

In  the  illustration  a  mercury  tube  is  suspended  between 
the  poles  of  the  magnet  and  connected  by  heavy  pressure 
tubing  with  a  duplex  vacuum  pump,  by  which  the  pressure 
of  the  mercury  vapor,  illuminated  by  the  discharge  of  an 
induction  coil,  can  be  reduced  as  desired.  The  current 
required  for  the  magnet  is  supplied  from  a  large  storage 
battery  in  an  adjoining  building.  This  battery  is  the  prin- 
cipal source  of  current  for  most  of  the  apparatus  on  the 
annular  pier;  an  alternating  current,  required  for  certain 
experiments,  is  obtained  from  a  generator  in  the  power-house. 

It  would  be  tedious  to  describe  in  detail  all  of  the  appa- 
ratus. It  includes  arrangements  for  studying  the  spark 
spectra  of  metals  in  air  and  in  liquids ;  arc  spectra  in  gases 
at  high  or  low  pressure;  flame  spectra,  for  which  a  Bunsen 
burner  and  an  oxyhydrogen  blow-pipe  are  required;  vacuum 
tube  spectra;  etc.  A  small  electric  furnace  permits  the  phe- 
nomena of  anomalous  dispersion  to  be  observed  in  the  vapors 
of  sodium  and  other  metals  which  melt  at  low  temperatures. 
The  auxiliary  apparatus  includes  a  special  pump  capable  of 
compressing  gases  up  to  pressures  of  three  thousand  pounds 
to  the  square  inch;  an  induction  coil,  giving  a  16-inch 
spark ;  X-ray  apparatus  for  the  study  of  the  effect  of  X-rays 
on  the  radiation  of  gases  and  vapors;  a  small  heliostat,  to 
supply  sunlight;  etc.  All  of  the  work  on  the  solar  image 
is  done  in  the  Snow  telescope  house,  but  sunlight  is  fre- 
quently required  in  the  laboratory,  to  give  a  solar  spectrum 
for  comparison  with  the  laboratory  spectra. 

Let  us  now  return  to  the  problem  of  explaining  the 
strengthening  and  weakening  of  the  solar  lines  in  sun-spot 
spectra.  As  already  remarked,  there  was  reason  to  suspect 
that  reduced  temperature  might  be  the  effective  cause  of 
these  changes.  Accordingly,  the  spectrum  of  iron  was 


A  STUDY  or  SUN-SPOTS  159 

photographed  by  Gale  and  Adams  in  the  electric  arc,  first 
with  a  large  current  (15  amperes),  and  then  with  a  small 
current  (2  amperes).  It  was  found  that  most  of  the  lines 
that  are  strengthened  in  spots  are  relatively  strengthened  in 
the  2-ampere  arc,  while  most  of  the  lines  that  are  weakened 
in  spots  are  also  weakened  in  this  arc.  Furthermore,  the 
majority  of  the  lines  showed  no  change  of  intensity,  which 
is  also  the  case  with  most  of  the  iron  lines  in  sun-spots. 
Similar  results  were  obtained  with  titanium,  vanadium,  chro- 
mium, manganese,  and  other  metals  represented  in  spots. 

The  next  question  was  to  determine  whether  the  metallic 
vapors  in  the  2-ampere  arc  are  certainly  cooler  than  in  the 
30-ampere  arc.  This  is  by  no  means  an  easy  thing  to  decide, 
on  account  of  various  complicating  elements  that  may  not 
appear  at  first  sight.  However,  it  was  a  simple  matter  to 
compare  the  spectrum  of  the  long  flame  which  extends  out 
from  the  arc  with  that  of  the  core  of  the  arc  between  the 
carbon  poles.  As  the  outer  part  of  the  flame  is  undoubtedly 
much  cooler  than  the  core  of  the  arc,  the  effect  of  decreased 
temperature  should  be  apparent  here.  The  results  confirmed, 
in  the  most  complete  manner,  those  obtained  by  reducing 
the  current.  In  other  words,  in  passing  from  the  hot  core 
of  the  arc  to  the  cooler  flame,  changes  in  the  relative  inten- 
sities of  the  lines  of  the  various  metals,  similar  to  those 
observed  in  comparing  the  solar  spectrum  with  the  sun-spot 
spectrum,  were  found.  It  thus  seemed  probable  that  the 
modified  relative  intensities  of  the  lines  in  spots  might  be  the 
result  of  a  local  reduction  in  temperature  of  the  solar  vapors. 

However,  it  is  not  known  precisely  what  part  electrical 
phenomena  in  the  arc  may  play  in  producing  the  character- 
istic radiations  of  the  vapors.  Indeed,  opinions  have  differed 
so  much  on  this  subject  that  some  of  the  ablest  physicists 
ascribe  the  observed  line  intensities  entirely  to  the  electrical 
conditions  of  the  arc,  and  do  not  admit  that  temperature 


160  STELLAR  EVOLUTION 

changes  can  have  any  influence  upon  them.  Thus  the 
results  so  far  obtained  would  not  be  accepted  as  proof  that 
the  spot  vapors  are  at  a  lower  temperature  than  the  corre- 
sponding vapors  in  the  Sun's  reversing  layer.  It  remained 
to  be  seen  whether  simple  reduction  of  temperature,  under 
conditions  which  excluded  any  possible  influence  of  electrical 
effects,  would  be  competent  to  change  the  relative  intensities 
of  the  lines  in  the  same  way  as  passage  from  the  core  to  the 
flame  of  the  arc  had  done. 

The  simplest  way  of  testing  this  was  to  inclose  the  metal 
in  question  within  a  carbon  or  graphite  tube  (chosen  because 
of  its  power  to  withstand  very  high  temperatures),  and  to 
heat  this  tube  by  a  powerful  electric  arc  playing  on  its  outer 
walls.  Under  these  conditions,  since  the  vapors  are  not 
observed  within  the  electric  arc,  but  are  separated  from  the 
flame  of  the  arc  by  the  walls  of  the  carbon  tube,  it  should 
be  possible  to  determine  the  effect  of  change  of  temperature 
on  the  relative  intensities  of  the  lines. 

As  the  dynamo  on  Mount  Wilson  was  not  adequate  to 
supply  the  electric  power  (50  kilowatts)  desired  for  this 
work,  the  furnace  was  erected  in  the  Pasadena  laboratory 
of  the  Solar  Observatory.  As  in  an  electric  furnace  used 
by  Moissan,  the  arc  was  produced  between  two  large  carbon 
poles,  in  a  box  with  carbon  walls,  surrounded  by  a  large  mass 
of  magnesite,  inclosed  in  a  sheet-iron  case.  Running  longi- 
tudinally through  the  carbon  box,  and  between  the  poles  of 
the  arc,  a  carbon  tube  containing  the  metal  was  placed.  This 
tube  extended  out  through  the  walls  of  the  furnace,  so  that 
light  from  the  hot  vapors  seen  through  its  open  end  could 
be  focused  on  the  slit  of  a  Littrow  spectrograph  of  18  feet 
focal  length  (similar  to  the  one  used  with  the  Snow  telescope 
in  photographing  spot  spectra). 

With  this  furnace  it  did  not  prove  to  be  possible  to 
vaporize  titanium  and  vanadium,  but  the  test  was  made  for 


A  STUDY  OF  SUN-SPOTS  161 

chromium  and  iron.  The  relative  intensities  of  the  lines  of 
these  metals  were  found  to  be  very  nearly  the  same  as  in 
the  flame  of  the  arc.  In  other  words,  the  lines  which  are 
strengthened  in  passing  from  the  core  of  the  arc  to  the  flame 
are  also  strengthened  in  passing  from  the  core  of  the  arc  to  the 
electric  furnace.  Moreover,  even  after  the  arc  which  heated 
the  carbon  tube  in  the  furnace  had  been  extinguished,  the 
still  glowing  vapors  continued  to  give  a  spectrum  in  which 
the  lines  strengthened  in  sun-spots  were  relatively  strong. 

But  the  proof  is  not  yet  complete.  For,  with  the  facili- 
ties available,  it  was  not  possible  to  vary  the  temperature  in 
the  furnace  through  a  sufficient  range  to  produce  undoubted 
changes  in  the  relative  intensities  of  the  lines.  Therefore  it 
might  be  argued  that  the  increased  intensity  in  the  core  of 
the  arc  of  some  lines,  and  the  decreased  intensity  of  others, 
are  due  to  electrical  phenomena,  and  not  to  increased  tem- 
perature. The  inference  was  strong  that  reduced  tempera- 
ture was  the  deciding  factor  in  determining  the  relative 
intensities  of  the  lines,  since  it  is  common  to  the  flame  of 
the  arc  and  to  the  furnace,  and  since  electrical  effects  were 
excluded  in  the  latter.  But  the  laboratory  work  cannot 
furnish  an  absolute  proof,  unless  it  should  become  possible, 
through  increase  in  the  temperature  of  the  furnace,  to  pro- 
duce spectra  in  which  the  relative  intensities  of  the  lines  are 
the  same  as  in  the  case  of  sun-spots.  Experiments  are  now 
in  progress  with  this  end  in  view. 

Fortunately,  however,  there  are  other  sources  of  infor- 
mation to  which  we  may  appeal.  In  the  reversing  layer, 
oxygen  exists  in  the  presence  of  such  substances  as  iron  and 
titanium.  Now,  it  is  well  known  that  this  can  be  true  only 
under  conditions  of  very  high  temperature.  Hence,  if  the 
metallic  vapors  in  sun-spots  are  actually  cooler  than  the 
vapors  outside  of  spots,  the  reduction  in  temperature  may 
be  sufficient  to  permit  the  oxygen  to  enter  into  combination 


162  STELLAR  EVOLUTION 

with  some  of  the  metals  present.  Titanium  oxide,  in  partic- 
ular, is  capable  of  resisting  a  very  high  temperature,  which 
would  immediately  dissociate  an  oxide  of  iron.  Is  there  any 
evidence,  then,  that  titanium  oxide  exists  in  sun-spots? 

Thanks  to  the  excellent  photographs  of  spot  spectra  ob- 
tained with  the  aid  of  the  Snow  telescope,  this  question  is 
easily  answered.  Titanium  oxide  gives  a  very  characteristic 
fluted  spectrum,  consisting  of  bands  in  which  the  numerous 
lines  lie  closer  and  closer  together  until  they  terminate  in 
definite  "heads."  Fig.  2,  Plate  LXXIV,  shows  some  of 
these  titanium  oxide  flutings  in  the  extreme  red  end  of  the 
spectrum,  as  photographed  (on  specially  sensitized  plates) 
in  the  outer  flame  of  the  electric  arc.  The  photograph  is 
a  negative;  i.  e.,  the  lines  which  are  bright  in  the  arc  are 
shown  dark,  to  facilitate  comparison  with  the  dark  lines 
in  the  photograph  of  the  spot  spectrum,  shown  just  above 
the  titanium  oxide  spectrum.  It  will  be  seen  at  a  glance 
that  each  of  the  heads  of  the  fluting  is  represented  in 
the  spot,  and  that  a  great  number  of  the  fine  lines  which 
make  up  the  fluting  also  agree  in  position  with  correspond- 
ing spot  lines.  The  spot  spectrum  contains  many  lines 
not  represented  in  the  arc,  which  are  due  to  substances 
other  than  titanium  oxide.  The  arc  spectrum  also  contains 
a  few  lines  due  to  impurities,  which  are  not  present  in  the 
spot.  Nevertheless,  the  general  agreement  is  so  perfect  that 
the  presence  of  the  titanium  oxide  bands  in  spot  spectra 
cannot  be  doubted.  Several  other  bands  belonging  to  the 
same  substance  are  also  represented  in  our  photographs  of 
spot  spectra. 

The  identification  of  these  bands  by  Adams  would  seem 
to  leave  no  doubt  as  to  the  reduced  temperature  of  the  spot 
vapors.  The  objection  might  be  made,  it  is  true,  that  some 
question  exists  as  to  whether  these  bands  are  actually  due  to 
the  oxide,  since  there  is  some  reason  to  suppose  that  titanium 


A  STUDY  OF  SUN-SPOTS  163 

itself  is  capable  of  producing  them.  However,  the  molecule 
which  gives  them  rise  undoubtedly  differs  from  the  atom 
which  produces  the  line  spectrum  of  titanium.  In  laboratory 
experiments  the  flutings  become  more  and  more  conspicuous 
as  the  temperature  is  reduced,  suggesting  that  the  molecule 
is  broken  up  at  high  temperatures.  The  absence  of  the  flut- 
ings from  the  spectrum  of  the  Sun  sustains  this  inference. 
Moreover,  Fowler,  in  London,  has  since  found  some  of  the 
green  flutings  in  the  Mount  Wilson  photographic  map  of 
the  spot  spectrum  to  be  due  to  magnesium  hydride,  and 
Olmsted,  on  Mount  Wilson,  has  identified  some  of  the  red 
flutings  with  those  of  calcium  hydride. 

It  therefore  appears  to  be  true  that  the  vapors  which  con- 
stitute the  umbra  of  a  sun-spot  are  cooler  than  the  corre- 
sponding vapors  in  other  parts  of  the  Sun.  This  would 
readily  account  for  the  relative  intensities  of  the  spectral 
lines  and  for  the  comparative  darkness  of  sun-spots.  But  the 
cause  of  such  a  reduction  of  temperature  is  yet  to  be  deter- 
mined. Knowledge  of  the  comparatively  low  temperature  of 
spot  vapors  at  once  permits  us,  however,  to  discard  various 
spot  theories  which  postulate  very  high  temperatures,  and  to 
attack  the  question  of  the  true  meaning  of  sun-spots  in  an 
intelligent  manner. 

In  order  to  facilitate  the  spectroscopic  study  of  sun- 
spots,  a  preliminary  photographic  map  of  the  spot  spectrum 
has  been  issued  by  the  Solar  Observatory.  This  consists  of 
twenty-six  sections,  each  covering  one  hundred  Angstrom 
units  of  the  spectrum,  the  whole  map  extending  from  wave- 
length 4600  to  wave-length  7200.  In  enlarging  the  original 
negatives,  Ellerman  photographed  each  section  on  a  sensitive 
plate,  moved  up  and  down  (in  the  direction  of  the  spectral 
lines)  during  the  exposure.  This  process  widened  the  narrow 
spot  spectrum,  and  rendered  visible  many  slight  changes  in 
the  relative  intensities  of  lines  which  would  otherwise  escape 


164  STELLAR  EVOLUTION 

notice.     Beside  each  strip  of  the  spot  spectrum  the  normal 
solar  spectrum  is  given  for  comparison  (Plate  LXXVI). 

The  information  derived,  as  explained  above,  from  solar 
and  laboratory  investigations  applies  not  only  to  the  Sun. 
If,  by  cooling  in  some  degree  the  vapors  lying  within  a 
limited  area  on  the  solar  surface,  the  spectrum  is  changed 
in  the  manner  illustrated  in  sun-spots,  it  should  follow  that 
if  the  entire  Sun,  or  a  star  like  the  Sun,  were  cooled  in  the 
same  degree,  its  spectrum  would  resemble  that  of  a  sun-spot. 
Our  ideas  of  stellar  evolution  are  based  on  the  belief  that 
stars  exist  in  all  stages  of  development  and  differing  greatly 
in  temperature.  If  our  inference  be  correct,  we  should  find, 
among  the  stars  which  have  passed  by  continued  cooling 
beyond  the  solar  stage,  some  in  whose  spectra  spot  lines 
appear.  The  next  chapter  explains  how  this  test  has  been 
applied. 


CHAPTER  XVIII 
STELLAR  TEMPERATURES 

THE  advantages  of  great  resolving  power  in  spectroscopic 
work  have  been  mentioned  in  previous  chapters.  In  the  case 
of  the  Sun  the  amount  of  light  at  our  disposal  is  so  abundant 
that  grating  spectroscopes  of  very  high  dispersion  can  be 
used  without  difficulty.  The  degree  in  which  the  light  is 
weakened  by  dispersion  will  be  appreciated  when  it  is  remem- 
bered that  the  light  entering  the  spectroscope  through  a  slit 
one-thousandth  of  an  inch  in  width  is  spread  out  into  a  spec- 
trum many  feet  in  length.  In  the  case  of  the  stars,  however, 
only  a  small  amount  of  light  is  at  our  disposal,  and  for  this 
reason  the  spectroscopes  employed  have  always  been  much 
inferior  in  dispersion  to  those  used  for  solar  research.  The 
interpretation  of  stellar  spectra  is  thus  rendered  difficult, 
since  several  closely  adjacent  lines  may  be  compressed  into 
one.  If,  then,  we  are  to  learn  the  true  relative  intensities  of 
stellar  lines,  in  order,  for  example,  to  make  certain  of  any 
apparent  analogy  with  sun-spots,  we  must  find  means  of 
studying  stellar  spectra  with  a  dispersion  as  great  as  that 
used  for  solar  observations. 

A  difficulty  which  does  not  exist  in  visual  observations 
has  an  important  bearing  on  the  nature  of  the  spectroscopes 
required  for  such  work.  If  it  were  possible  to  see  the  spec- 
trum of  a  star  to  good  advantage,  a  high  resolving  power 
could  be  obtained  with  a  spectroscope  of  moderate  dimen- 
sions, supplied  with  a  powerful  grating.  But,  for  two  principal 
reasons,  photographic  methods  are  almost  exclusively  used 
in  stellar  spectroscopy.  In  the  first  place,  except  in  the  case 
of  a  few  of  the  brightest  stars,  the  smaller  details  of  stellar 

165 


166  STELLAR  EVOLUTION 

spectra  cannot  be  seen,  on  account  of  the  faintness  of  the 
light.  In  the  second  place,  the  unsteadiness  of  the  image, 
due  to  atmospheric  disturbances,  causes  the  extremely  narrow 
spectrum  to  nicker  so  seriously  as  to  prevent  any  refined 
work.  This  nickering,  however,  has  no  effect  upon  the  photo- 
graphic plate,  which  merely  sums  up  all  of  the  light  it 
receives  during  the  exposure.  Moreover,  by  prolonging  the 
exposure,  a  spectrum  too  faint  to  be  seen  can  be  recorded 
photographically.  In  all  modern  work  of  precision,  there- 
fore, photographs  of  stellar  spectra  are  substituted  for  visual 
observations. 

But  the  photographic  plate  has  a  granular  structure,  due 
to  the  fact  that  it  is  made  up  of  silver  grains,  which  can  be 
separately  distinguished  with  a  microscope.  On  account  of 
this  granular  structure  of  the  plate  the  details  of  the  image 
are  imperfectly  recorded,  so  that  no  advantage  results  from 
the  use  of  high  powers  when  examining  the  plate.  If  the 
visual  image  could  be  well  seen  at  the  spectroscope,  an 
increase  of  magnification  (attained  by  the  use  of  a  suitable 
eye-piece)  would  separate  all  lines  within  the  resolving  power 
of  the  prisms  or  grating.  On  the  photographic  plate,  how- 
ever, the  images  of  these  lines  may  lie  so  close  together  that 
they  appear  as  one,  and  cannot  be  separated  by  magnification. 
What  is  needed,  in  order  to  realize  photographically  the  full 
resolving  power  of  the  prisms  or  grating  employed,  is  a  spec- 
troscope of  such  length  that  the  closest  lines  that  could  be 
distinguished  visually  are  so  far  separated  as  to  be  independ- 
ently recorded,  in  spite  of  the  effect  of  the  silver  grains. 

The  powerful  grating  spectrograph  used  by  Rowland  in 
his  study  of  the  solar  spectrum  has  a  focal  length  of  21  feet. 
Photographs  made  with  a  spectrograph  of  this  size  show 
nearly  all  the  lines  that  can  be  separately  distinguished  in 
visual  observations  with  the  same  instrument.  Obviously  it 
would  be  out  of  the  question  to  attach  such  a  spectrograph, 


STELLAR  TEMPERATURES  167 

or  even  an  equally  powerful  one  of  the  more  compact  Littrow 
type,  to  the  end  of  a  movable  telescope  tube.  Moreover, 
the  very  high  dispersion  would  demand,  in  the  case  of 
stars,  exposures  prolonged  for  many  nights.  Temperature 
changes,  or  the  slightest  flexure  of  the  apparatus  during  the 
exposure,  would  shift  the  position  of  the  lines  on  the  plate 
and  thus  destroy,  by  producing  a  blurred  image,  all  the 
advantages  afforded  by  large  spectrographs. 

Such  instruments  as  the  three-prism  spectrograph  of  the. 
Potsdam  Astrophysical  Observatory,  the  Mills  spectrograph 
of  the  Lick  Observatory,  and  the  Bruce  spectrograph  of  the 
Yerkes  Observatory,  give  beautifully  defined  photographs  of 
stellar  spectra,  from  the  measurement  of  which  the  motions 
of  stars  in  the  line  of  sight  are  determined  with  great  pre- 
cision. For  most  classes  of  work  such  spectrographs  could 
hardly  be  surpassed.  Nevertheless,  the  necessary  limitations 
of  resolving  power  and  focal  length  in  these  instruments 
prevents  them  from  separating  many  of  the  lines  resolved  by 
Rowland  in  his  studies  of  the  solar  spectrum.  It  is  evidently 
to  be  greatly  desired  that  the  spectra  of  a  few  of  the  brightest 
stars,  at  least,  be  photographed  with  spectrographs  as  power- 
ful as  Rowland's.  In  order  to  accomplish  this  the  spectro- 
graph must  be  fixed  in  position  on  a  massive  pier,  and  main- 
tained at  a  constant  temperature  throughout  the  exposure. 

To  test  the  feasibility  of  this,  and  to  decide  whether  a 
spectrograph  of  high  dispersion  could  advantageously  be 
used  with  a  60-inch  reflecting  telescope  (p.  228),  a  grating 
spectrograph  of  13  feet  focal  length  has  been  tried  with 
the  Snow  telescope.  This  instrument  was  mounted  on  the 
triangular  stone  pier  (p.  133,  Fig.  5)  in  the  spectroscope 
house  of  the  Snow  telescope.  The  pier  is  inclosed  in  a  room 
so  constructed  that  the  fluctuations  of  temperature  within 
it  are  very  slight.  The  6 -inch  Rowland  plane  grating  was 
mounted  so  as  to  form  the  front  wall  of  a  cubical  metallic 


168  STELLAR  EVOLUTION 

box  containing  water.  An  extremely  delicate  thermostat,  con- 
sisting of  a  bulb  containing  saturated  ether  vapor  immersed 
in  the  water,  caused  a  column  of  mecury  to  make  or  break 
an  electric  circuit  if  the  temperature  of  the  water  varied  as 
much  as  a  hundredth  of  a  degree.  When  the  temperature  fell 
by  this  amount,  a  relay  turned  on  the  current  of  two  incan- 
descent lamps  immersed  in  the  liquid.  The  heating  produced 
by  the  lamps  raised  the  temperature,  and  the  current  was 
then  automatically  cut  off.  The  water  was  constantly  stirred 
by  small  propellers  driven  by  an  electric  motor.  In  this  way 
the  grating,  which  is,  of  course,  the  most  sensitive  part  of  the 
apparatus,  was  kept  at  an  almost  perfectly  constant  tempera- 
ture throughout  the  exposure. 

Arcturus,  on  account  of  its  yellowish  color  and  the  charac- 
ter of  its  spectrum,  has  long  been  considered  to  represent  a 
stage  of  stellar  development  somewhat  advanced  beyond  that 
of  the  Sun.  As  its  spectral  lines  show  its  chemical  com- 
position to  be  practically  the  same  as  that  of  the  Sun,  a 
reduction  in  temperature,  due  to  cooling  continued  beyond 
the  solar  stage,  should,  on  the  hypothesis  developed  in  the 
last  chapter,  cause  its  spectrum  to  resemble  that  of  a  sun- 
spot.  Accordingly,  the  spectrum  of  Arcturus  was  photo- 
graphed with  the  Snow  telescope  and  the  grating  spectro- 
graph. 

Because  of  the  great  dispersion,  an  exposure  of  five  hours, 
which  was  all  that  could  be  given  on  a  single  night,  was 
entirely  insufficient.  In  fact,  an  exposure  continued  for  five 
nights  in  succession,  and  aggregating  twenty -three  hours, 
was  required.  During  all  this  time  it  was  essential  that  the 
temperature  of  the  grating  remain  practically  constant,  and 
that  none  of  the  parts  of  the  spectrograph  be  displaced  by 
any  cause.  For  this  reason  the  observer  did  not  enter  the 
constant-temperature  room  after  the  exposure  Was  started, 
but  merely  brought  the  star  to  the  slit  of  the  spectrograph 


STELLAR  TEMPERATURES  169 

each  night,  and  maintained  it  there,  by  watching  the  star 
image  reflected  from  the  slit  jaws,  and  correcting  any  slight 
deviations  in  its  position.  The  same  process  was  repeated 
from  night  to  night,  until  the  exposure  was  completed. 

In  these  first  experiments  the  possibilities  of  the  method 
were  not  fairly  tested,  on  account  of  some  imperfections  in 
the  apparatus.  The  Snow  telescope  was  designed  for  solar 
work,  and  is  not  well  adapted  for  stellar  observations.  More- 
over, work  in  progress  on  the  telescope  house  caused  some 
vibration  of  the  piers,  which  doubtless  affected  the  definition. 
Nevertheless,  the  resulting  photographs  are  sufficiently  good 
to  show  that  this  method,  when  properly  carried  out  with 
the  60-inch  reflector,  should  give  a  few  stellar  spectra  not 
essentially  inferior  to  the  best  obtained  in  solar  work.  The 
60-inch  reflector  will  collect  about  six  times  as  much  light 
as  the  Snow  telescope,  and  the  exposure  time,  for  the  same 
dispersion,  will  be  decreased  in  about  this  ratio.  Thus  the 
spectrum  of  Arcturus  should  be  photographed  with  the 
grating  used  for  the  present  work  in  about  four  hours.  As 
subsequent  experiments  with  the  Snow  telescope  showed  that 
large  prisms  can  be  used  to  much  better  advantage  than  the 
grating  for  stellar  spectra,  this  exposure  time,  for  the  same 
dispersion,  will  be  still  further  reduced.  In  the  case  of  the 
60-inch  reflector,  the  dispersion  will  be  increased  sufficiently 
to  make  the  scale  of  the  spectrum  about  the  same  as  that  of 
Rowland's  solar  spectrum  photographs. 

Plate  LXXVII  shows  a  portion  of  the  Arcturus  spectrum 
thus  photographed,  in  comparison  with  spot  and  solar  spectra. 
Barring  some  exceptions,  which  require  further  study,  it  will 
be  noticed  that  the  spectrum  resembles  the  spot  spectrum 
more  closely  than  it  does  the  solar  spectrum.1  On  account 

1  In  comparing  these  spectra,  changes  of  intensity  should  be  noted  with  reference 
to  adjoining  (unaffected)  lines  in  the  same  spectrum.  Unavoidable  differences  of 
absolute  intensity  in  the  photographs  prevent  a  satisfactory  comparison,  unless  this 
precaution  be  observed. 


170  STELLAR  EVOLUTION 

of  the  imperfections  of  the  Arcturus  photograph,  many  of 
the  lines  are  shown  with  less  contrast  than  they  would  ex- 
exhibit  in  a  really  good  negative.  However,  the  illustration 
should  be  sufficient  to  indicate  the  important  bearing  of  spot 
spectra  on  the  question  of  stellar  temperatures. 

The  earliest  classification  of  stellar  spectra  was  that  of 
Secchi,  who  distinguished  four  principal  types:  I,  spectra 
of  white  and  bluish-white  stars,  like  Sirius,  which  contain 
broad  and  strong  hydrogen  and  calcium  lines,  and  but  few 
lines,  narrow  and  comparatively  faint,  of  other  elements;  II, 
spectra  of  yellowish-white  stars,  like  the  Sun;  III,  spectra 
of  red  stars,  containing  a  very  characteristic  series  of  bands, 
not  identified  by  Secchi;  IV,  spectra  of  another  class  of  red 
stars,  containing  the  strongly  marked  bands  of  carbon.  The 
bands  in  the  spectra  of  stars  of  Secchi's  third  type  were 
finally  identified  by  Fowler,  who  showed  that  they  are  due 
to  titanium  oxide.  In  view  of  the  presence  of  these  same 
bands  in  spot  spectra  (Fig.  2,  Plate  LXXIV),  it  becomes 
interesting  to  inquire  whether  the  lines  in  stellar  spectra  of 
this  type  also  resemble  those  in  sun-spots. 

The  brilliant  red  star  Beielgeuze  (a  Orionis)  which  pre- 
sents so  striking  a  contrast  with  the  bluish  star  Rigel,  in 
the  constellation  of  Orion,  is  a  good  representative  of  the 
third  type.  It  was  accordingly  selected  to  test  the  question. 
A  dense  flint  glass  prism  belonging  to  the  5-foot  spectrohelio- 
graph  was  substituted  for  the  grating  in  the  large  stellar 
spectrograph  of  the  Snow  telescope,  and  the  thermostat  was 
modified  so  as  to  control  the  temperature  of  the  air  surround- 
ing the  prism.  In  this  way  the  spectrum  of  a  Orionis  was 
photographed  by  Adams,  with  a  total  exposure  of  seven  hours 
on  two  consecutive  nights.  The  work  was  done  during  the 
rainy  season,  and  clouds,  followed  by  continuous  bad  weather, 
cut  short  the  exposure  on  the  second  night,  and  prevented  the 
observations  from  being  continued.  The  plate,  while  not 


STELLAR  TEMPERATURES  171 

strong  enough  to  be  of  the  best  quality,  is  nevertheless 
sufficiently  good  to  serve  for  the  purpose  of  a  general  com- 
parison. It  was  found  that  essentially  all  of  the  lines  are 
stronger  than  in  the  Sun,  and  that  lines  which  are  strength- 
ened in  spots  are  much  more  decidedly  strengthened  in  a 
Orionis  than  lines  unaffected  in  spots.  In  fact,  the  relative 
strengthening  is  much  more  marked  in  the  case  of  this  star 
than  in  the  spots  themselves,  probably  indicating  that  its 
temperature  is  lower.  As  the  titanium  oxide  flutings  form  a 
conspicuous  feature  of  the  spectrum  of  a  Orionis,  and  are  also 
present  in  sun-spots,  the  evidence  appears  to  be  practically 
complete.  More  detailed  investigations  will  undoubtedly 
reveal  various  discrepancies,  due  to  differences  in  chemical 
composition  or  physical  condition.  Nevertheless,  it  may  be 
said,  in  general,  that  the  resemblance  between  the  spectra  of 
sun-spots  and  those  of  third-type  stars  is  so  close  as  to  indi- 
cate that  the  same  cause  is  controlling  the  relative  intensities 
of  many  lines  in  both  instances.  This  cause,  as  the  laboratory 
work  indicates,  is  to  be  regarded  as  reduced  temperature. 

Thus  we  have  been  led,  through  the  study  of  certain 
phenomena  of  our  typical  star,  the  Sun,  and  through  their 
interpretation  by  laboratory  experiments,  to  the  considera- 
tion of  the  general  question  of  stellar  temperatures.  Let  us 
now  inquire  whether  other  independent  methods  can  be 
applied  to  determine  these  temperatures,  dealing  first  with 
the  possibility  of  measuring  directly  the  heat  radiation  of 
stars. 

The  early  experiments  of  Huggins  and  Stone  failed,  for 
lack  of  suitable  apparatus,  to  detect  the  exceedingly  small 
degrees  of  heat  which  reach  us  from  stellar  sources.  Even 
Boys  was  no  more  successful  in  1888,  though  he  concen- 
trated the  stellar  radiations  on  his  newly  invented  radio- 
micrometer,  which  would  show  1000000  of  a  degree  rise  of 
temperature.  With  the  sensitiveness  used,  TTJnnnr  °^  tne 


172  STELLAR  EVOLUTION 

heat  received  by  his  telescope  mirror  from  the  full  Moon  could 
be  detected.  Yet  the  brightest  stars  produced  no  certain 
effect.  As  the  result  of  this  work,  Boys  was  convinced  that 
no  star  sends  us  as  much  heat  as  would  be  received  from  a 
candle  at  a  distance  of  1.7  miles,  if  there  were  no  atmos- 
pheric absorption. 

The  subject  of  stellar  heat  was  investigated  by  E.  F. 
Nichols  at  the  Yerkes  Observatory,  in  1898  and  1900.  The 
radiometer  employed  as  the  heat-measuring  apparatus  con- 
sisted of  two  circular  vanes  of  mica,  each  about  one-twelfth 
of  an  inch  in  diameter,  attached  to  the  opposite  ends  of  a 
delicate  cross-arm  of  drawn  glass,  cemented  to  a  whip  of  fine 
drawn  glass  about  one  and  one-quarter  inches  long.  To  the 
lower  end  of  this  system  a  minute  mirror,  made  by  silvering 
a  fragment  of  very  thin  microscope  cover-glass,  was  attached, 
and  the  whole  was  suspended  by  a  very  fine  quartz  fiber  in 
a  vacuum  chamber.  This  radiometer  was  mounted  on  a  pier 
in  the  coelostat  room  of  the  Yerkes  Observatory.  A  coelostat 
reflected  the  starlight  to  a  24-inch  mirror  of  8  feet  focal 
length,  which  concentrated  the  stellar  rays  upon  one  of  the 
vanes,  after  entering  the  radiometer  case  through  a  window 
of  fluorite,  which  is  very  transparent  to  heat  radiations.  By 
observing  a  scale  reflected  in  the  small  mirror  attached  to 
the  radiometer  suspension,  the  deflection  of  the  vane,  which 
indicated  the  heating  effect  of  the  stellar  rays,  could  be 
measured.  In  this  way  it  was  found  that  Arcturus  sends  us 
about  as  much  heat  as  would  be  received  from  a  candle  six 
miles  away,  if  there  were  no  absorption  in  the  atmosphere. 
Vega  has  less  than  half  the  thermal  intensity  of  Arcturus. 

The  extraordinary  sensitiveness  of  the  apparatus  employed 
may  be  illustrated  by  some  observations  of  a  candle  2,500 
feet  from  the  observatory.  Heat  from  this  candle,  when 
concentrated  on  the  radiometer  vane  of  the  24-inch  mirror, 
gave  a  deflection  of  about  sixty-two  scale  divisions.  On  one 


STELLAK  TEMPERATUBES  173 

occasion  the  assistant  extinguished  the  candle  and  placed 
his  head  in  front  of  it  when  the  signal  was  given,  instead  of 
uncovering  the  flaine.  The  deflection  caused  by  the  heat 
radiation  of  his  face,  at  a  distance  of  2,500  feet,  was  twenty- 
five  scale  divisions!  With  no  atmospheric  absorption,  the 
number  of  candles  in  a  group  at  a  distance  of  sixteen  miles 
could  be  determined  from  the  average  of  a  series  of  meas- 
urements of  their  total  heat  radiation. 

As  Arcturus  and  Vega  appear  about  equally  bright  to  the 
eye,  the  greater  heat  radiation  of  the  former  star  indicates 
that  it  sends  out  a  larger  proportion  of  the  long  (red)  waves. 
If  neither  star  possessed  an  absorbing  atmosphere,  it  might 
then  be  concluded  that  Arcturus  is  cooler  than  Vega,  but  so 
much  larger  in  angular  diameter,  when  seen  from  the  Earth, 
as  to  be  fully  as  bright  as  Vega,  and  to  send  us  more  than 
twice  as  much  heat.  However,  since  we  know  that  the 
absorbing  atmosphere  of  stars  like  Arcturus  is  much  denser 
than  that  of  stars  like  Vega,  this  conclusion  would  not 
hold.  We  are  therefore  not  in  a  position  to  judge  from 
these  experiments  as  to  the  relative  temperatures  of  these 
stars. 

Lockyer  has  recently  endeavored  to  determine  the  relative 
temperatures  of  stars  by  comparing  their  spectra,  when 
photographed  under  similar  conditions,  in  order  to  learn 
which  of  two  stars  sends  us  the  greater  proportion  of  violet 
light.  In  accordance  with  a  well-known  law,  the  proportion 
of  violet  light  emitted  by  a  luminous  body  increases  as  the 
temperature  rises.  By  measuring  the  position  of  maximum 
intensity  in  the  spectrum  of  a  star,  it  should  thus  be  possible 
to  determine  its  temperature.  Unfortunately,  however,  as 
already  remarked,  no  absolutely  safe  conclusions  can  be  based 
upon  a  test  of  this  kind.  Stars  with  dense  atmospheres  must 
appear  red  in  color,  no  matter  what  their  temperature,  as  com- 
pared with  stars  whose  atmospheres  are  much  less  dense.  For 


174  STELLAR  EVOLUTION 

we  have  here  just  such  a  condition  of  things  as  we  observe  in 
the  setting  Sun,  which  appears  red  simply  because  the  violet 
rays  are  more  highly  absorbed  by  our  atmosphere  than  the 
red  rays.  It  seems  to  be  true  that  the  older  and  cooler  stars 
have  denser  atmospheres  than  the  younger  and  hotter  ones. 
It  is  thus  probable  that  the  stars  whose  spectra  contain  the 
greater  proportion  of  red  light  actually  are  cooler  than  those 
in  which  the  violet  light  is  relatively  stronger.  But  the 
fact  remains  that  we  are  not  warranted  in  basing  determina- 
tions of  stellar  temperatures  on  measurements  which  so 
obviously  depend  upon  the  effect  of  atmospheres  of  unknown 
density.  We  will  return  to  this  question  of  stellar  tempera- 
tures in  a  further  consideration  of  the  classification  of  stars 
(chap,  xx ). 


CHAPTER   XIX 
THE  NEBULAR  HYPOTHESIS 

IN  the  preceding  chapters  we  have  seen  how  the  study  of 
stellar  evolution  depends  primarily  upon  the  most  accurate 
knowledge  we  can  obtain  of  the  Sun,  regarded  as  a  typical 
star.  We  have  also  examined  certain  methods  of  observing 
solar,  stellar  and  laboratory  phenomena,  and  have  taken 
advantage  of  the  opportunity  afforded  by  the  peculiarities  of 
Sun-spot  spectra  to  illustrate  the  mutual  dependence  of  these 
various  means  of  research.  In  passing  to  certain  of  the  more 
general  considerations  underlying  our  subject,  we  may  now 
examine  some  of  the  principal  hypotheses  which  have  been 
offered  to  account  for  the  development  of  solar  and  stellar 
systems. 

Passing  over  the  important  speculations  of  Kant,  and  the 
conclusions  drawn  by  Herschel  from  his  extensive  observa- 
tions, we  reach  the  nebular  hypothesis  of  Laplace.  This  cele- 
brated explanation  of  the  origin  of  the  solar  system  has  domi- 
nated the  world's  thought  since  the  very  date  of  its  publication. 
The  eminence  of  its  author,  and  the  unique  value  of  his 
great  work  on  celestial  mechanics,  led  to  the  immediate 
acceptance  of  his  ideas,  even  when  advanced  in  speculative 
form  and  without  the  support  of  mathematical  analysis. 
The  greatest  physicists  and  astronomers  of  the  nineteenth 
century  have  given  the  weight  of  their  approval  to  the 
nebular  hypothesis,  and  all  calculations  as  to  the  age  of  the 
Sun  have  been  based  upon  it.  When  viewing  it  in  the  light 
of  recent  destructive  criticism,  we  must  not  forget  the  value 
of  Laplace's  speculations  in  directing  thought  and  in  seek- 
ing to  account,  by  a  single  generalization,  for  a  host  of 

175 


176  STELLAR  EVOLUTION 

observed  phenomena.  Nor  must  we  overlook  his  remark 
that  the  hypothesis  was  presented  "with  the  distrust  which 
should  be  inspired  by  everything  that  is  not  the  result  of 
observation  or  calculation."  The  widespread  and  favorable 
influence  exerted  by  the  hypothesis  on  the  intellectual  life 
of  the  nineteenth  century  cannot  be  destroyed  by  recent 
developments.  In  the  same  way,  the  beneficial  effect  of 
Darwin's  work  on  organic  evolution  would  remain,  even  if 
the  hypothesis  of  natural  selection  were  forced  from  its  place 
by  that  of  mutation. 

As  the  original  statement  of  the  nebular  hypothesis  is 
not  easily  accessible  to  every  reader,  it  seems  desirable  to 
include  here  a  free  translation  of  Note  VII,  at  the  end  of 
Laplace's  Exposition  du  sysi&me  du  monde.  A  few  para- 
graphs, dealing  with  more  technical  details,  are  omitted,  but 
all  of  the  essential  features  are  retained. 

In  seeking  to  trace  the  cause  of  the  original  motions  of  the 
planetary  systems,  the  following  five  phenomena,  enumerated  in 
the  last  chapter  (of  Laplace's  book),  are  available:  the  motions  of 
the  planets  in  the  same  direction  and  nearly  in  the  same  plane;  the 
motions  of  the  satellites  in  the  same  direction  as  the  planets;  the 
motions  of  rotation  of  these  different  bodies  and  of  the  Sun  in  the 
same  direction  as  their  orbital  motions,  and  in  but  slightly  different 
planes;  the  small  eccentricity  of  the  orbits  of  planets  and  satellites; 
finally,  the  great  eccentricity  of  comets'  orbits,  as  though  their 
inclination  had  been  left  to  chance. 

So  far  as  I  am  aware,  Buffon  is  the  only  one  who  has  endeavored, 
since  the  discovery  of  the  true  system  of  the  world,  to  trace  the 
origin  of  the  planets  and  their  satellites.  He  supposes  that  a 
comet,  falling  upon  the  Sun,  drove  from  it  a  torrent  of  matter, 
which  reunited  at  a  distance  in  several  globes,  varying  in  size  and 
in  distance  from  the  Sun;  these  globes,  having  become  opaque  and 
solid  by  cooling,  are  the  planets  and  their  satellites. 

Laplace  then  goes  on  to  show  that,  although  this  hypoth- 
esis might  account  for  the  first  of  the  five  phenomena 


THE  NEBULAE  HYPOTHESIS  177 

mentioned  above,  the  others  could  not  be  explained  by 
it.  In  seeking  to  discern  their  true  cause,  he  continues  as 
follows : 

Whatever  be  its  nature,  since  it  has  produced  or  directed  the 
motions  of  the  planets,  it  must  have  embraced  all  of  these  bodies, 
and,  in  view  of  the  prodigious  distances  that  separate  them,  it 
could  only  have  been  a  fluid  of  immense  extent.  In  order  to  give 
them  a  nearly  circular  motion  about  the  Sun,  in  the  same  direction, 
the  fluid  must  have  surrounded  this  body  like  an  atmosphere.  The 
consideration  of  planetary  motions  thus  leads  us  to  think  that,  as 
the  result  of  excessive  heat,  the  solar  system  originally  extended 
beyond  the  orbits  of  all  the  planets,  and  that  it  contracted  by  suc- 
cessive steps  to  its  present  limits. 

In  the  assumed  primitive  condition  of  the  Sun,  it  resembled 
those  nebulae  which  are  shown  by  the  telescope  to  be  composed  of 
a  more  or  less  brilliant  nucleus,  surrounded  by  nebulosity  which, 
in  condensing  toward  the  surface  of  the  nucleus,  transforms  it  into 
a  star.  If,  by  analogy,  we  conceive  of  all  the  stars  being  formed 
in  this  manner,  we  may  imagine  their  earlier  nebular  state,  itself 
preceded  by  other  states,  in  which  the  nebular  matter  was  more 
and  more  diffuse,  the  nucleus  being  less  and  less  luminous.  By 
going  back  as  far  as  possible,  we  thus  arrive  at  a  nebulosity  so 
diffuse  that  its  existence  could  hardly  be  suspected. 

Philosophical  observers  have  long  been  impressed  with  the 
peculiar  distribution  of  certain  stars  visible  to  the  naked  eye. 
Mitchel  has  remarked  on  the  improbability  that  the  stars  of  the 
Pleiades,  for  example,  could  have  been  compressed  within  the 
narrow  limits  which  inclose  them  by  mere  chance,  and  he  has 
hence  concluded  that  this  group  of  stars,  and  similar  groups  in  the 
heavens,  are  the  effects  of  an  original  cause  or  of  a  general  law  of 
nature.  These  groups  are  the  necessary  result  of  the  condensation 
of  nebulae  having  several  nuclei;  for  it  is  evident  that,  if  the  nebu- 
lar matter  were  continually  attracted  by  these  various  nuclei,  they 
would  ultimately  form  a  group  of  stars  like  that  of  the  Pleiades. 
The  condensation  of  nebulae  having  two  nuclei  will  similarly  form 
stars  lying  very  close  together,  and  revolving  about  one  another, 
like  the  double  stars  whose  motions  have  already  been  observed. 

But  how  has  the  solar  atmosphere  determined  the  motions  of 
rotation  and  of  revolution  of  the  planets  and  satellites  ?  If  these 


178  STELLAE  EVOLUTION 

bodies  had  penetrated  deeply  into  this  atmosphere,  its  resistance 
would  have  caused  them  to  fall  upon  the  Sun.  We  may  thus  con- 
jecture that  the  planets  were  formed  at  its  successive  limits,  by  the 
condensation  of  zones  of  vapors  which  the  Sun,  in  cooling,  must 
have  abandoned  in  the  plane  of  its  equator. 

Let  us  recall  the  results  given  in  a  preceding  chapter.  The 
atmosphere  of  the  Sun  could  not  have  extended  out  indefinitely. 
Its  limit  was  the  point  where  the  centrifugal  force,  due  to  its 
motion  of  rotation,  balanced  the  attraction  of  gravitation.  Now,  as 
cooling  contracted  the  atmosphere  and  condensed  at  the  surface  of 
the  Sun  the  molecules  lying  near  it,  the  motion  of  rotation  acceler- 
ated. For,  from  the  law  of  areas,  the  sum  of  the  areas  described 
by  the  radius  vector  of  each  molecule  of  the  Sun  and  of  its  atmos- 
phere, when  projected  on  the  plane  of  its  equator,  being  always 
the  same,  the  rotation  must  be  more  rapid  when  these  molecules 
approach  the  center  of  the  Sun.  The  centrifugal  force  due  to  this 
motion  thus  becoming  greater,  the  point  where  it  equals  the  weight 
is  nearer  the  Sun.  If  we  then  adopt  the  natural  supposition  that 
the  atmosphere  extended,  at  some  period,  to  an  extreme  limit,  it 
must  have  left  behind,  in  cooling,  the  molecules  situated  at  this 
limit  and  at  the  successive  limits  produced  by  the  acceleration  of 
the  Sun's  rotation.  These  abandoned  molecules  must  have  con- 
tinued to  revolve  around  the  Sun,  since  their  centrifugal  force  was 
balanced  by  their  weight.  But  since  this  equilibrium  did  not 
obtain  in  the  case  of  the  atmospheric  molecules  in  higher  latitudes, 
their  weight  caused  them  to  approach  the  atmosphere  as  it  con- 
densed, and  they  did  not  cease  to  belong  to  it  until  this  motion 
brought  them  to  the  equator. 

Let  us  now  consider  the  zones  of  vapor  successively  left  behind. 
To  all  appearances  these  zones  should  form,  by  their  condensation 
and  the  mutual  attraction  of  their  molecules,  various  concentric 
rings  of  vapor  revolving  around  the  Sun.  The  mutual  friction  of 
the  molecules  of  each  ring  should  have  accelerated  some  and 
retarded  others,  until  they  had  all  acquired  the  same  angular 
velocity.  Thus  the  linear  velocities  of  the  molecules  farthest  from 
the  center  of  the  Sun  must  have  been  the  greatest.  The  following 
cause  would  also  contribute  toward  the  production  of  this  difference 
of  velocity.  The  molecules  farthest  from  the  Sun,  which,  through 
the  effects  of  cooling  and  condensation,  came  together  to  form  the 
outer  part  of  the  ring,  always  described  areas  proportional  to  the 


THE  NEBULAR  HYPOTHESIS  179 

time,  since  the  central  force  which  controlled  them  was  constantly 
directed  toward  the  Sun.  This  constancy  of  areas  requires  that 
the  velocity  increase  as  the  molecules  move  inward.  It  is  evident 
that  the  same  cause  must  have  diminished  the  velocity  of  those 
molecules  which  moved  outward  to  form  the  inner  edge  of  the  ring. 

If  all  the  molecules  of  a  ring  of  vapor  continued  to  condense 
without  separating,  they  would  finally  form  a  liquid  or  solid  ring. 
But  the  uniformity  which  this  formation  demands  in  all  parts  of 
the  ring,  and  in  their  rate  of  cooling,  must  have  rendered  this 
phenomenon  extremely  fare.  Thus  the  solar  system  offers  only  a 
single  example  of  it,  that  of  the  rings  of  Saturn.  In  almost  all 
cases  each  ring  of  vapor  must  have  broken  into  several  masses 
which,  having  only  slightly  different  velocities,  continued  to  revolve 
at  the  same  distance  around  the  Sun.  These  masses  must  have 
assumed  a  spheroidal  form,  with  a  motion  of  rotation  correspond- 
ing in  direction  with  that  of  their  revolution,  since  their  inner 
molecules  had  smaller  linear  velocities  than  their  outer  molecules; 
they  thus  formed  as  many  planets  in  a  vaporous  state.  But  if  one 
of  them  had  possessed  sufficient  power  of  attraction  to  bring  all  the 
others  successively  together  about  its  own  center,  the  vaporous  ring 
would  thus  have  been  transformed  into  a  single  spheroidal  mass  of 
vapor,  revolving  about  the  Sun  and  rotating  in  a  direction  corre- 
sponding to  that  of  its  revolution.  This  latter  case  has  been  the 
most  common  one.  Nevertheless,  the  solar  system  offers  an 
example  of  the  first  case  in  the  four  minor  planets  which  lie  between 
Jupiter  and  Mars  ;  unless  we  suppose,  in  agreement  with  M.  Olbers, 
that  they  originally  formed  a  single  planet  broken  up  by  a  violent 
explosion  into  several  parts  having  different  velocities. 

Now,  if  we  follow  the  changes  which  ultimate  cooling  must 
have  produced  in  the  vaporous  planets  whose  formation  we  have 
just  pictured,  we  shall  witness  the  production,  at  the  center  of  each, 
of  a  nucleus  which  continues  to  develop  through  the  condensation 
of  the  atmosphere  surrounding  it.  In  this  state  the  planet  exactly 
resembles  the  Sun  in  its  primitive  nebular  condition.  Cooling 
must  thus  have  produced,  at  the  various  limits  of  its  atmosphere, 
phenomena  similar  to  those  we  have  described ;  that  is  to  say,  rings 
and  satellites  revolving  around  its  center  in  the  direction  of  its 
motion  of  rotation,  and  turning  in  the  same  direction  upon  them- 
selves. The  symmetrical  distribution  of  Saturn's  rings  about  its 
center  and  in  the  plane  of  its  equator  naturally  results  from  this 


180  STELLAR  EVOLUTION 

hypothesis,  and  would  be  inexplicable  without  it.  These  rings 
seem  to  me  ever-present  proofs  of  the  original  extension  of 
Saturn's  atmosphere  and  of  its  successive  retreats.  Thus  the 
singular  phenomena  of  the  slight  eccentricity  of  the  orbits  of  the 
planets  and  satellites,  the  small  inclination  of  these  orbits  to  the 
solar  equator,  the  identity  in  direction  of  the  motions  of  rotation 
and  revolution  of  all  these  bodies  with  that  of  the  solar  rotation: 
flow  from  our  hypothesis  and  give  it  great  probability. 

If  the  solar  system  had  been  formed  with  perfect  regularity, 
the  orbits  of  the  bodies  which  compose  it  would  have  been  circles 
whose  planes,  like  those  of  the  various  equators  and  rings,  would 
have  coincided  with  the  plane  of  the  solar  equator.  But  it  may  be 
conceived  that  the  endless  varieties  which  must  have  existed  in  the 
temperature  and  density  of  the  various  parts  of  these  great  masses 
produced  the  eccentricity  of  their  orbits  and  the  deviation  of  their 
motions  from  the  plane  of  this  equator. 

In  our  hypothesis,  comets  are  strangers  to  the  planetary  system. 
In  considering  them,  as  we  have  done,  to  be  small  nebulae  wander- 
ing from  system  to  system,  and  formed  by  the  condensation  of 
nebular  matter  distributed  with  such  profusion  throughout  the 
universe,  we  perceive  that,  when  they  arrive  in  the  region  of 
space  where  the  solar  attraction  is  predominant,  it  forces  them 
to  describe  elliptical  and  hyperbolic  orbits.  But  their  motions 
being  equally  possible  in  all  directions,  they  must  move  indifferently 
in  all  directions  and  at  all  inclinations  to  the  ecliptic ;  which  is  in 
agreement  with  observation.  Thus  the  condensation  of  nebular 
matter,  by  which  we  have  just  explained  the  motions  of  rotation 
and  revolution  of  the  planets  and  satellites  in  the  same  direction, 
and  in  planes  differing  but  slightly,  also  explains  why  the  motions 
of  comets  do  not  agree  with  this  general  law. 

Laplace,  after  discussing  the  great  eccentricity  of  comets' 
orbits,  as  bearing  on  the  nebular  hypothesis,  continues  as 
follows : 

If  certain  comets  entered  the  atmospheres  of  the  Sun  and 
planets  during  the  formative  period  they  must  have  fallen  upon 
these  bodies,  after  pursuing  spiral  paths.  The  result  of  their  fall 
would  be  to  cause  the  planes  of  the  orbits  and  the  equators  of  the 
planets  to  deviate  from  the  solar  equator. 


THE  NEBULAE  HYPOTHESIS  181 

If  in  the  zones  left  behind  by  the  solar  atmosphere  there  were 
molecules  too  volatile  to  combine  among  themselves  or  with  the 
planets,  they  must  have  continued  to  revolve  about  the  Sun.  They 
would  thus  give  rise  to  such  an  appearance  as  that  of  the  zodiacal 
light,  without  offering  appreciable  resistance  to  the  various  bodies 
of  the  planetary  system,  either  because  of  their  extreme  rarity,  or 
because  their  motion  is  very  nearly  the  same  as  that  of  the  planets 
which  they  encounter. 

A  close  examination  of  all  the  details  of  the  solar  system  adds 
still  further  to  the  probability  of  our  hypothesis.  The  original 
fluidity  of  the  planets  is  clearly  indicated  by  the  flattening  of  their 
figure,  in  conformity  with  the  laws  of  mutual  attraction  of  their 
molecules ;  furthermore,  it  is  proved  in  the  case  of  the  Earth  by 
the  regular  diminution  of  weight  from  the  equator  to  the  poles. 
This  condition  of  original  fluidity,  to  which  we  are  led  by  astronomi- 
cal phenomena,  should  show  itself  in  the  phenomena  of  natural 
history.  But,  to  perceive  it  there,  it  is  necessary  to  take  into 
account  the  immense  variety  of  combinations  formed  by  all  ter- 
restrial substances  mingled  together  in  a  state  of  vapor,  when  the 
reduction  of  temperature  permitted  their  elements  to  unite  among 
themselves.  It  is  also  necessary  to  consider  the  enormous  changes 
that  this  fall  of  temperature  must  have  brought  about  successively 
within  the  Earth  and  upon  its  surface,  in  all  formations,  in  the 
constitution  and  the  pressure  of  the  atmosphere,  in  the  ocean,  and 
in  the  bodies  which  it  held  in  solution.  Finally,  consideration 
should  be  given  to  violent  disturbances,  such  as  great  volcanic 
eruptions,  which  must  have  modified,  at  various  epochs,  the  regu- 
larity of  these  changes.  Geology  studied  from  this  point  of  view, 
which  unites  it  to  astronomy,  will  acquire  precision  and  certainty 
in  many  particulars. 

Although  the  nebular  hypothesis  received  almost  universal 
acceptance,  objections  and  difficulties  were  brought  forward 
at  various  times  during  the  nineteenth  century.  The  criti- 
cisms of  Babinet  and  Kirkwood  were  followed  by  the  argu- 
ments of  Faye,  who  concluded  that  the  planets,  if  developed 
from  the  ring-system  of  Laplace,  should  rotate  in  the  oppo- 
site direction.  Laplace  had  assumed  that  the  rings  which 
were  to  form  the  planets  revolved  like  solid  bodies,  their 


182  STELLAR  EVOLUTION 

outer  edge  traveling  faster  than  the  inner  one.  This  would 
have  involved  forward  rotation  of  the  planets,  as  now  observed. 
But  such  a  condition  of  things  could  not  have  occurred — the 
rings,  split  asunder  by  the  forces  acting  upon  them,  must 
have  followed  Kepler's  laws,  which  would  require  the  inner 
edge  to  move  the  faster.  The  rings  of  Saturn,  held  up  by 
Laplace  as  a  striking  illustration  of  his  views,  were  shown 
by  Maxwell  in  1859  to  be  composed  of  small  bodies  like 
meteorites.  This  was  the  result  of  a  mathematical  demon- 
stration that  the  rings,  if  solid,  would  fly  to  pieces.  It  was 
confirmed  by  Keeler,  in  1895,  by  one  of  the  most  beautiful 
applications  of  the  spectroscope  ever  made.  According  to 
Doppler's  principle,  the  position  of  a  line  in  the  spectrum 
of  a  moving  body  depends  upon  the  velocity  of  the  mo- 
tion. This  is  true,  even  when  the  light  is  reflected  from 
the  surface  of  the  moving  body,  after  being  received  from 
the  Sun.  If  the  inner  edge  of  Saturn's  ring  is  moving 
faster  than  the  outer  edge,  the  lines  in  the  spectrum  of  the 
ring  should  be  increasingly  bent  toward  the  violet  (on  the 
approaching  side  of  the  planet)  or  toward  the  red  (on 
the  receding  side),  in  passing  from  the  outer  toward  .the 
inner  edge.  Keeler's  photograph  of  Saturn's  spectrum 
shows  this  to  be  the  case.  Thus  we  have  certain  proof  that 
Saturn's  rings  are  made  up  of  meteorites,  each  moving  at  the 
velocity  a  satellite  would  have  at  the  same  distance  from  the 
planet. 

In  spite  of  these  and  kindred  objections,  the  nebular 
hypothesis,  at  least  in  its  general  outlines,  retained  its  com- 
manding position  until  subjected  to  a  searching  test  insti- 
tuted by  Chamberlin  and  Moulton.  The  principal  arguments 
brought  together  in  Moulton's  paper,  entitled  "An  Attempt 
to  Test  the  Nebular  Hypothesis  by  an  Appeal  to  the  Laws  of 
Dynamics,"1  and  in  the  discussion  of  the  question  in  Volume 

1  Astrophysical  Journal,  Vol.  XI  (1900),  p.  103. 


THE  NEBULAR  HYPOTHESIS  183 

II  of  Chamberlin  and  Salisbury's  Geology,  are  briefly  sum- 
marized below. 

In  the  paper  just  referred  to,  Moulton  defines  the  nebu- 
lar hypothesis  in  much  more  general  terms  than  Laplace 
employed.  In  other  words,  in  order  to  make  the  test  as 
complete  as  possible,  he  assumes  that  the  original  nebula 
might  consist  of  a  gas  or  of  a  swarm  of  meteorites,  since 
Darwin  had  proved  mathematically  that  the  properties  of 
gases  may  be  fulfilled  in  a  meteoroidal  swarm.  Moulton's 
discussion,  moreover,  does  not  insist  upon  the  assumption  of 
a  very  high  temperature,  since  the  progress  of  knowledge 
has  shown  that  the  present  heat  of  the  Sun  may  be  accounted 
for  as  a  result  of  the  contraction  of  a  nebula  originally  at  a 
low  temperatare.  Finally,  the  breaking-up  of  the  nebula  is 
not  limited  to  the  abandonment  of  rings,  but  is  considered 
to  include  possible  division  by  some  fission  process,  the 
separated  portions  having  contracted  to  form  the  planets  and 
satellites. 

The  fact  that  the  revolutions  of  certain  satellites,  such  as 
those  of  Uranus  and  Neptune,  are  in  a  retrograde  direction, 
while  the  planes  of  the  orbits  of  the  four  satellites  of  Uranus 
are  almost  perpendicular  to  the  plane  of  the  planet's  orbit, 
is  an  old  argument  against  the  nebular  hypothesis.  While 
the  former  difficulty  could  easily  be  overcome,  the  great 
inclination  of  the  orbits  of  these  satellites  and  that  of  Nep- 
tune seems  to  be  directly  opposed  to  Laplace's  views.  In  the 
second  place,  the  masses  of  the  various  planets,  as  well  as  the 
densities  of  the  rings  from  which  they  are  supposed  to  be 
formed,  are  shown  to  be  entirely  out  of  harmony  with  what 
the  hypothesis  would  lead  us  to  expect.  Again,  the  inner 
satellite  revolves  about  Mars  in  a  period  less  than  a  third  of 
the  planet's  rotation,  while  the  hypothesis  would  require  its 
velocity  to  be  much  less  than  that  of  the  planet's  surface. 
Darwin  has  shown  that  the  friction  of  solar  tides  might  have 


184  STELLAR  EVOLUTION 

retarded  the  rotation  of  Mars,  without  affecting  the  satellite's 
motion.  But  Moulton  points  out  that  the  inner  edge  of 
Saturn 's  ring  completes  a  revolution  in  about  half  the  time 
of  Saturn's  rotation  period.  At  this  great  distance  from 
the  Sun,  the  very  small  tides  could  not  have  retarded  suf- 
ficiently the  rotation  period  of  Saturn,  unless  they  have 
been  operating  several  thousand  times  as  long  as  the  Martian 
tides.  An  attempt  to  ascribe  the  effect  to  the  satellites  of 
Saturn  proves  equally  futile. 

Moulton  next  endeavors  to  answer  the  question  whether 
the  supposed  initial  conditions  could  have  developed  into  the 
existing  system.  We  know  that  the  molecules  of  a  gas  are 
moving  about  at  velocities  which  increase  with  the  tempera- 
ture. Near  the  surface  of  the  original  Laplacian  nebula  the 
velocities  of  the  molecules,  in  the  case  of  such  light  elements 
as  hydrogen,  would  be  so  great  that  the  molecules  would 
overcome  the  power  of  gravitation  and  be  dispersed  in  space. 
It  would,  therefore,  be  difficult  to  account  for  the  abundant 
supplies  of  this  gas  now  observed  on  the  Sun.  A  stronger 
objection  is  afforded  through  the  application  of  these  prin- 
ciples to  the  planets.  It  is  easy  to  calculate,  through  the 
known  velocities  of  gaseous  molecules  and  the  masses  of  the 
planets,  the  power  of  each  planet  to  retain  an  atmosphere. 
It  is  also  possible  to  determine  with  the  spectroscope  whether 
atmospheres  exist  on  the  planets.  Working  in  this  way, 
Moulton  shows  the  improbability  that  the  diffuse  Earth -Moon 
ring,  with  its  low  power  of  attraction,  could  have  held  any  of 
the  atmospheric  gases  or  water  vapor,  when  such  concentrated 
bodies  as  the  Moon  and  Mercury  are  unable  at  the  present 
time  to  hold  atmospheres. 

As  we  know  the  masses  of  the  Sun  and  planets,  the 
average  density  of  the  original  nebula,  when  it  extended  to 
the  orbit  of  Neptune,  can  be  approximately  calculated. 
Moulton  finds  this  to  be  about -j-y-nnj- TOO" o~o oir  °^ that °^  water. 


THE  NEBULAR  HYPOTHESIS  185 

In  this  extraordinarily  rare  nebula,  whether  truly  gaseous  or 
meteoroidal,  it  is  shown  that  -matter  would  have  been  left 
behind  continually  and  that  the  formation  of  separate  rings 
would  be  impossible  —  a  conclusion  reached  by  Kirkwood  in 
1869.  Moulton  thinks  it  equally  certain  that  a  large  mass 
could  not  have  been  detached  by  any  fission  process.  Fur- 
thermore, even  if  a  ring  had  been  formed,  he  shows  it  to  be 
utterly  improbable  that  its  matter  could  have  been  drawn 
together  into  a  planet. 

Some  of  the  above  conclusions  may  perhaps  be  open  -to 
question,  but  the  final  argument  seems  to  be  unanswerable. 
It  is  a  well-known  principle  of  dynamics  that  the  moment  of 
momentum  of  a  system  of  bodies  not  under  the  action  of 
external  forces  is  constant.  The  moment  of  momentum  is 
defined  by  the  sum  of  the  products  of  the  masses  of  all  the 
particles  by  their  velocities  and  by  their  distances  from  the 
center  of  the  system.  This  quantity  should  remain  abso- 
lutely unchanged,  whether  the  system  be  in  the  form  of  a 
nebula  occupying  the  whole  of  Neptune's  orbit,  or  a  group 
of  planets  revolving  around  the  Sun.  Making  his  assump- 
tions in  such  a  way  as  to  be  most  favorable  to  the  nebular 
hypothesis,  Moulton  obtains  the  following  results  for  the 
moment  of  momentum: 

When  the  nebula  extended  to  Neptune's  orbit  M=32.176 

When  the  nebula  extended  to  Jupiter's  orbit  M=13.250 

When  the  nebula  extended  to  the  Earth's  orbit  M=  5.690 

When  the  nebula  extended  to  Mercury's  orbit  M=  3.400 

In  the  system  at  present  M=  0.151 

Thus,  instead  of  remaining  constant,  the  moment  of 
momentum  is  shown  to  decrease  rapidly  and  irregularly. 
In  spite  of  the  precautions  taken  to  favor  the  nebular 
hypothesis  as  much  as  possible,  the  moment  of  momentum 
of  the  original  system  comes  out  213  times  that  of  the 
present  solar  system. 


186  STELLAR  EVOLUTION 

The  papers  of  Chamberlin  and  Moulton  contain  other 
serious  criticisms  based  upon  the  study  of  the  moment  of 
momentum  of  the  system,  and  raise  various  additional  difficul- 
ties. Thus  the  attenuated  state  of  the  rock-forming  substances 
of  the  Earth  in  the  Earth-Moon  ring  would  probably  have 
resulted  in  their  condensation  into  solid  particles.  Again,  no 
nebulae  closely  resembling  the  annulated  solar  nebula  have 
yet  been  discovered.  Without  going  further  into  details,  and 
without  necessarily  admitting  the  finality  of  all  the  above 
arguments,  it  can  hardly  be  denied  that  Laplace's  idea  of  the 
development  of  the  solar  system  must  be  reconstructed  or 
abandoned.  It  remains  to  be  seen  what  can  be  substituted 
for  it.  Two  attempts  in  this  direction  will  be  described  in  a 
later  chapter. 


CHAPTER   XX 
STELLAR  DEVELOPMENT 

THE  nebular  hypothesis,  as  outlined  in  the  last  chapter, 
presents  a  picture  of  the  development  of  a  planetary  system 
like  our  own.  In  testing  it,  recourse  may  be  had  both  to 
theoretical  investigations  and  to  observations  of  various 
kinds,  particularly  of  nebulae,  which  may  throw  light  on 
the  earlier  stages  of  the  process  of  condensation.  It  must 
be  remembered  that  planets  comparable  in  size  with  the 
members  of  our  solar  system  would  be  quite  invisible  at 
the  distances  of  the  stars.  However,  in  the  study  of  stellar 
evolution  we  are  concerned  primarily  with  stars,  rather  than 
with  the  planets  that  may  accompany  them.  It  is  neverthe- 
less evident  that  the  two  questions  cannot  be  considered 
independently,  since  the  details  of  the  processes  that  result 
in  the  formation  of  planets  must  be  of  the  highest  impor- 
tance in  researches  on  the  development  of  the  central  suns 
of  which  they  may  have  formed  a  part. 

Herschel,  whose  mind  was  always  occupied  with  the  prob- 
lem of  the  structure  of  the  universe  and  the  formation  of  its 
individual  members,  thought  he  perceived  in  the  nebulae 
evidences  of  growth  and  development.  He  supposed  that 
the  cloud  forms,  of  irregular  structure,  which  extend  over 
vast  regions  of  the  heavens,  represent  the  earliest  and  most 
rudimentary  condition  of  stellar  life.  Condensation  toward 
a  center,  brought  about  by  the  action  of  gravity,  would  be 
shown  in  such  a  cloud  by  increased  brightness.  Latest  in  the 
line  of  nebular  existence  Herschel  placed  the  planetary  nebu- 
lae, in  whose  symmetrical  forms  he  saw  illustrated  some  such 
condition  as  Laplace  postulated  for  the  primitive  solar  system. 

187 


188  STELLAR  EVOLUTION 

The  mystery  of  the  planetary  nebulae  still  remains  un- 
solved, but  evidence  is  lacking  that  they  represent  a  more 
advanced  state  than  such  irregular  cloud  masses  as  the 
Great  Nebula  in  Orion.  Indeed,  it  must  be  admitted  that 
the  accumulation  of  observations,  principally  through  the 
aid  of  photography,  has  rendered  the  problem  of  nebular 
development  more  complex  than  it  appeared  to  Sir  William 
Herschel.  Thousands  of  nebulae,  entirely  unknown  to  him, 
have  been  brought  to  our  knowledge  through  improvements 
in  telescope  design  and  the  aid  of  the  sensitive  plate.  These 
range  in  character  from  immense  luminous  tracts,  such  as 
are  shown,  intermingled  with  stars,  in  photographs  of  the 
Milky  Way,  to  the  definite  outlines  and  highly  suggestive 
structure  of  the  spiral  nebulae.  Of  all  objects  in  the  heavens 
these  latter  most  strongly  suggest  the  operation  of  some 
process  of  development.  But  not  a  single  object  of  this  type 
was  known  to  Herschel,  and  even  to  this  day  their  enormous 
distance  from  the  Earth  has  prevented  the  detection  of  any 
changes  in  form,  which  might  point  to  the  explanation  of 
their  origin.1 

If  we  follow  Herschel,  and  consider  the  simplest  case  of 
nebular  development,  we  may  suppose  that  through  loss  of 
heat  by  radiation  a  portion  of  a  nebulous  mass  begins  to 
condense  toward  a  center.  Although  still  wholly  gaseous, 
and  showing  few  points  of  difference  from  an  ordinary  nebula, 
we  may  regard  such  an  object  as  representing  the  first  period 
in  the  life  of  a  star.  In  the  heart  of  the  Orion  nebula,  Plate 
XXI,  are  four  small  stars,  which  constitute  the  well-known 
Trapezium.  Situated  as  they  are  in  this  enormous  mass 
of  gas,  it  is  not  difficult  to  picture  them  as  centers  of  con- 
densation, toward  which  the  play  of  gravitational  forces 
tends  to  concentrate  the  gases  of  the  nebula.  It  might 
therefore  be  expected  that  stars  in  this  early  stage  of  growth 

1  See  chap.  xxi. 


STELLAR  DEVELOPMENT  189 

would  show,  through  the  spectroscopic  analysis  of  their  light, 
some  evidence  of  relationship  with  the  surrounding  nebula. 
Now,  this  is  precisely  what  the  spectroscope  has  demon- 
strated. Not  only  these  stars,  but  many  others  in  the  con- 
stellation of  Orion,  are  shown  by  the  spectroscope  to  contain 
the  same  gases  that  constitute  the  nebula.  Moreover,  they 
also  partake  of  its  motion  through  space.  Finally,  Frost  and 
Adams  have  demonstrated  the  interesting  fact  that  some  of 
these  stars  are  actually  moving  in  orbits  about  dark  com- 
panions situated  in  the  very  heart  of  the  nebula.  Since  the 
orbital  velocities  of  the  moving  stars  are  very  high,  it  thus 
seems  probable  that  the  matter  which  constitutes  the  Great 
Nebula  in  Orion  is  exceedingly  tenuous,  offering  little  resist- 
ance to  motion  within  it. 

Other  examples  of  direct  relationship  between  stars  and 
surrounding  masses  of  nebulae  might  be  mentioned,  but 
this  one  will  suffice  for  our  present  purpose.  We  must  now 
consider  what  changes  in  color  and  in  spectrum  accompany 
the  further  development  of  the  star  as  it  continues  to  lose 
heat  through  radiation. 

Fraunhofer  was  the  first,  in  the  opening  years  of  the 
nineteenth  century,  to  observe  the  spectra  of  the  stars.  The 
simple  method  he  employed,  which  consisted  in  placing  a 
prism  over  the  object-glass  of  a  telescope,  has  since  become, 
through  the  skill  and  energy  of  Pickering,  a  wonderfully 
effective  agent  for  the  wholesale  study  of  stellar  spectra. 
To  Fraunhofer  the  differences  he  perceived  when  comparing 
the  spectra  of  different  stars  were  of  no  meaning,  since 
the  work  of  Kirchhoff  had  not  yet  been  done.  But 
the  photographs  made  under  Pickering's  direction  at  the 
Harvard  College  Observatory  now  tell  a  remarkable  story 
to  the  initiated.  In  making  these  photographs,  a  large 
prism  is  mounted  in  front  of  the  object-glass  of  a 
(refracting)  telescope,  which  is  directed  to  a  field  of  stars 


190  STELLAR  EVOLUTION 

and  made  to  follow  its  apparent  motion  by  a  driving-clock. 
Under  these  conditions,  each  star-image  in  the  field  of  the 
telescope  is  drawn  out  into  a  spectrum,  which  falls  upon  a 
photographic  plate  at  the  focus.  If  the  rate  of  the  driving- 
clock  were  perfect,  each  of  these  spectra  would  be  extremely 
narrow,  and  the  "lines"  which  cross  it  might  not  be  per- 
ceptible. To  give  the  spectra  the  necessary  width,  the 
prism  is  set  with  its  refracting  edge  parallel  to  the  diurnal 
motion,  so  that  the  spectra  would  drift  on  the  photographic 
plate,  if  the  telescope  were  at  rest,  in  a  direction  at  right 
angles  to  their  length.  In  making  the  photographs,  the  rate 
of  the  driving-clock  is  slightly  altered,  so  that  the  drift  of 
the  spectra  during  the  exposure  is  just  sufficient  to  give  them 
the  desired  width.  Without  this  drift,  each  "line"  would 
be  merely  a  point  in  the  spectrum.  Plate  LXXIX  illustrates 
how  admirably  the  spectra  of  the  various  stars  in  the  field 
are  recorded,  and  brings  before  us  evidence  of  the  spectral 
diversity  which  is  supposed  to  characterize  the  different 
stages  of  stellar  growth. 

As  already  indicated  (chap,  xviii),  the  spectra  of  stars 
increase  in  complexity  as  the  cooling  process  continues.  The 
gaseous  nebulae  contain  a  few  bright  lines  in  their  spectra, 
the  most  conspicuous  one  of  which  belongs  to  a  gas  ("nebu- 
lum")  not  yet  discovered  on  the  Earth.  The  other  nebular 
lines  are  due  to  hydrogen  and  helium.  Those  stars  of  the 
"Orion  type"  which  appear  to  be  earliest  in  order  of  devel- 
opment contain  no  lines  except  those  of  hydrogen  and  helium, 
which  are  faint  and  very  broad  and  diffuse.  As  these  gases 
are  found  in  the  gaseous  nebulae,  and  as  the  relationship 
of  these  stars  to  surrounding  nebulous  matter  is  otherwise 
apparent,  there  is  every  reason  to  believe  that  they  represent 
the  earliest  phase  of  stellar  life.  The  stars  of  the  Trapezium, 
which  have  already  been  mentioned  as  organically  related  to 
the  Great  Nebula  of  Orion,  are  of  this  type.  "Orion"  stars, 


STELLAR  DEVELOPMENT  191 

which  appear  to  be  somewhat  further  developed,  show  lines 
of  magnesium,  silicon,  oxygen,  and  nitrogen,  in  addition  to 
those  of  hydrogen  and  helium  (Plate  LXXX). 

Next  in  order  of  evolution  appear  to  be  the  white,  or 
bluish-white,  stars  like  Sirius  (Fig.  1,  Plate  LXXXI).  The 
spectrum  of  Sirius  is  marked  by  broad  and  conspicuous 
hydrogen  lines,  associated  with  narrow  and  faint  lines  of 
iron,  sodium,  magnesium,  etc.  It  has  been  shown  by  investi- 
gations of  certain  pairs  of  stars,  in  which  the  two  components 
are  in  rapid  rotation  about  their  common  center  of  gravity, 
that  stars  like  Sirius  are  much  less  dense  than  the  Sun,  their 
specific  gravity  not  exceeding  that  of  water.  This,  of  course, 
•-is,  exactly  what  would  be  expected  in  an  early  stage  of  transi- 
tion from  a  gaseous  nebula  to  a  highly  Condensed  star. 

It  is  well  known  through  mathematical  demonstration 
that  the  condensation  of  such  a  mass  of  gas  as  that  in  which 
the  Sun  originated  must  involve  the  production  of  a  vast 
amount  of  heat.  Indeed,  the  present  solar  radiation  may  be 
accounted  for  by  supposing  that  the  Sun's  diameter  decreases 
about  400  feet  in  the  course  of  a  year.  The  seemingly 
paradoxical  fact  that  a  gaseous  mass,  through  loss  of  heat  by 
radiation,  will  actually  grow  hotter  as  long  as  it  remains  in  a 
gaseous  condition,  was  demonstrated  by  Lane  in  1870.  The 
point  is  that  the  heat  produced  by  shrinkage  is  more  than 
sufficient  to  compensate  for  the  loss  by  radiation.  Conse- 
quently, the  shrinking  mass  grows  hotter  as  long  as  it  remains 
purely  gaseous.  The  time  finally  comes,  however,  when  its 
outer  parts,  which  radiate  freely  into  space  and  are  not  pro- 
tected from  loss  by  outlying  masses  of  heated  matter,  are 
cooled  to  the  point  of  condensation.  That  is  to  say,  certain 
metallic  elements  present  in  a  state  of  vapor  condense  into 
clouds  made  up  of  minute  liquid  drops,  thus  resembling  our 
terrestrial  clouds,  which  are  caused  by  the  condensation  of 
water  vapor. 


192  STELLAR  EVOLUTION 

When  this  point  is  reached,  radiation  must  take  place 
mainly  from  the  surface  of  the  star.  This  would  result  in 
very  rapid  surface  cooling,  were  it  not  for  convection  cur- 
rents, which  rise  from  the  interior  and  supply  the  heat  lost 
by  radiation.  In  the  Sun  we  have  strong  evidence  of  the 
existence  of  such  currents,  which  are  represented  by  the 
bright  filaments  that  constitute  the  granulated  surface 
(chap,  xi),  and  by  the  minute  flocculi  illustrated  in  Plate 
XXXIX.  The  darker  spaces  (pores)  between  the  granulations 
probably  represent  the  cooler  descending  vapors.  The  denser 
vapors,  which  perhaps  occupy  these  darker  regions,  apparently 
lie  below  the  general  photospheric  level,  for  it  has  recently 
been  found  at  Mount  Wilson  that  the  spectrum  of  the  Sun's 
disk,  at  points  very  near  the  limb,  differs"  decidedly  from  the 
spectrum  at  the  center.  The  hazy  wings,  which  may  be  seen 
on  either  side  of  many  lines  photographed  at  the  Sun's  cen- 
ter, and  are  still  more  conspicuous  in  sun-spots,  are  greatly 
reduced  in  intensity  at  the  limb  (Plate  LXXXII).  This 
would  seem  to  indicate  that  at  the  center  of  the  Sun  we  are 
looking  down  into  the  regions  between  the  granulations,  to  a 
level  where  the  vapor  is  dense  enough  to  produce  the  winged 
lines.  Near  the  edge  of  the  Sun,  on  the  contrary,  we  look 
across  the  tops  of  the  bright  filaments,  and  therefore  fail  to 
receive  light  from  the  denser  vapors  below.  The  absorption 
of  the  higher  and  cooler  vapors  should  produce  a  change  in  the 
relative  intensities  of  the  lines  such  as  takes  place  in  sun- 
spots  (p.  159),  but  in  much  smaller  degree.  Observation 
shows  this  to  be  the  case,  but  there  is  by  no  means  a  strict 
parallel  between  the  two  classes  of  phenomena,  and  judg- 
ment must  be  reserved  for  the  present.  One  of  the  next 
steps  will  be  to  photograph  the  spectrum  of  a  pore,  if  so 
minute  an  object  can  be  separately  observed.  The  inves- 
tigation, when  completely  worked  out,  should  furnish  a 
searching  criterion  as  to  the  validity  of  the  hypothesis  of 


STELLAR  DEVELOPMENT  193 

reduced  temperature  in  spots,  and  as  to  the  cause  of  certain 
phenomena  in  the  Sun  and  stars. 

We  have  already  seen  (p.  173)  that  increased  density  of 
the  absorbing  atmosphere  tends  to  reduce  the  proportion  of 
violet  and  ultra-violet  rays,  and  thus  to  introduce  a  yellowish 
or  reddish  tinge  into  the  star's  light.  Such  stars  as  Sirius 
do  not  possess  dense  absorbing  atmospheres,  and  because 
of  this  fact  and  of  their  extremely  high  temperature,  their 
spectra  extend  far  into  the  ultra-violet. 

In  passing  from  these  white  stars  to  the  yellowish  stars, 
which  constitute  the  solar  class,  the  continued  process  of 
condensation  is  accompanied  by  the  production  of  an  absorb- 
ing atmosphere  similar  to  that  of  the  Sun.  Beginning  in 
the  ultra-violet,  the  absorption  becomes  more  and  more 
appreciable  as  the  solar  type  of  star  is  approached.  The 
decrease  of  intensity,  while  most  marked  in  the  ultra-violet 
region,  is  also  manifest  in  the  blue  and  violet  part  of  the 
spectrum,  whereas  the  red,  yellow,  and  green  are  not  greatly 
affected.  The  natural  result  is  a  change  of  color,  through 
a  deficiency  in  blue  light.  For  this  reason,  stars  of  the 
solar  class  are  yellowish  in  hue.  Langley  has  pointed 
out  that  the  Sun  would  appear  bluish-white,  if  its  absorbing 
atmosphere  were  removed.  Accompanying  this  change  of 
color  we  have  the  decreasing  strength  of  the  hydrogen  lines, 
and  the  increasing  strength  of  the  metallic  lines,  which 
become  very  numerous  in  Procyon  (Plate  LXXXI)  and  still 
more  so  in  the  Sun. 

As  already  remarked,  this  gradual  increase  of  atmospheric 
absorption  prevents  us  from  basing  conclusions  as  to  relative 
stellar  temperatures  on  the  position  of  the  maximum  of 
intensity  in  the  spectrum.  We  may  fall  back,  however, 
upon  comparisons  of  the  relative  intensities  of  certain 
lines,  just  as  was  done  in  the  study  of  sun-spots  described 
in  chap.  xvii.  This  method  of  classifying  stars  according 


194  STELLAR  EVOLUTION 

to  their  temperature  was  applied  by  Lockyer  many  years 
ago.  He  found  that  the  "enhanced  lines,"  which  are 
brightest  in  the  spark  spectrum  of  a  metal,  exist  alone  in 
certain  stars.  In  other  words,  the  arc  lines  of  the  same 
metal,  which  are  strong  at  the  lower  temperature  of  the  arc, 
and  feeble  or  absent  in  the  spark,  are  so  much  reduced  in 
intensity  in  these  stars  as  to  be  entirely  invisible. 

Lockyer's  contention  that  these  changes  of  relative  inten- 
sity afford  a  mode  of  classifying  stellar  spectra  on  a  temper- 
ature basis  was  denied  by  many  spectroscopists,  because  of 
the  possibility  that  such  changes  might  be  produced  in  stars 
by  different  electrical  conditions  rather  than  by  differences 
of  temperature.  The  results  obtained  in  our  laboratory 
imitation  of  sun-spot  phenomena,  however,  seem  to  favor 
the  view  that  a  temperature  classification  of  stars,  on  the 
basis  of  the  relative  intensities  of  lines,  is  perfectly  pos- 
sible. For  in  these  experiments  it  was  shown  that 
when  all  electrical  phenomena  are  excluded,  a  decrease  in 
temperature  of  the  radiating  vapors  is  accompanied  by  an 
increase  in  intensity  of  the  lines  that  are  strengthened  in 
sun-spots  and  in  red  stars.  Since  the  spark  lines  are  weak- 
ened -under  the  same  conditions,  and  since  conclusive  evi- 
dence of  comparatively  low  temperature  is  afforded  by  the 
presence  in  these  spectra  of  flutings  due  to  substances  which 
are  broken  up  at  the  higher  temperature  of  the* Sun,  the 
temperature  hypothesis  may  perhaps  be  taken  as  affording  a 
simple  basis  of  classification.  This  statement  is  not  made 
without  some  reservations,  however,  as  indicated  by  the 
remarks  at  the  end  of  this  chapter,  and  by  the  discussion 
of  Lockyer's  meteoritic  hypothesis  in  chap.  xxi.  Moreover, 
since  this  classification  takes  no  account  of  the  possible  effect 
of  mass  and  environment  on  spectral  type,  it  is  hardly  likely 
to  prove  adequate. 

Let  us  now  consider  the  phenomena  of  declining  stars, 


STELLAR  DEVELOPMENT  195 

which  have  passed  beyond  the  solar  stage  and  are  fading  into 
invisibility.  It  will  be  remembered  that  these  stars  are 
orange  or  red  in  color  and  that  they  may  be  divided  into  two 
classes,  similar  in  appearance  to  the  eye,  but  easily  dis- 
tinguishable with  the  aid  of  the  spectroscope.  The  first  of 
these  classes  (Secchi's  third  type)  includes  certain  bright 
stars,  such  as  the  red  Antares,  which  is  a  conspicuous 
object  in  the  southern  heavens  during  the  summer  months. 
The  second  class  (Secchi's  fourth  type)  has  no  brilliant 
representative.  Indeed,  the  brightest  stars  of  this  char- 
acter are  but  barely  discernible  by  the  naked  eye,  while  the 
great  majority  are  to  be  observed  only  with  the  aid  of  a 
telescope. 

In  the  spectroscope  both  classes  show  a  spectrum  vastly 
more  complicated  than  that  of  stars  in  an  earlier  stage  of 
growth.  The  broad  lines  of  hydrogen,  which  are  greatly 
reduced  in  intensity  in  the  Sun,  are  still  further  reduced  in 
the  red  stars.  In  fact,  the  dark  hydrogen  lines  have  in  cer- 
tain red  stars  given  place  to  bright  lines,  especially  in  the 
case  of  variable  stars,  whose  light  undergoes  regular  or 
irregular  fluctuations.  The  most  characteristic  feature  of 
the  red  stars,  however,  is  the  presence  in  their  spectra  of 
dark  bands  or  flutings.  In  third-type  stars  the  sharp  edges 
of  these  bands  lie  toward  the  violet,  while  on  the  red  side 
the  intensity  gradually  decreases.  These  bands  have  been 
found  by  Fowler  to  be  due  to  the  oxide  of  titanium,  which  may 
be  broken  up  at  the  higher  temperature  of  the  Sun,  but  exists 
in  the  spectra  of  sun-spots  (Plate  LXXIV) .  The  bands  of  the 
red  stars  of  Secchi's  fourth  type  face  in  the  opposite  direction, 
with  their  sharply  defined  boundaries  toward  the  red.  These 
bands,  as  Plates  LXXXIII  and  LXXXIV  illustrate,  are  due 
to  carbon  and  cyanogen.  Some  of  them  are  faintly  present 
in  the  Sun,  but  in  the  fourth-type  stars  they  are  much  more 
strongly  developed. 


196  STELLAR  EVOLUTION 

The  extensive  investigations  of  Yogel  and  Duner,  made 
visually,  have  given  us  much  information  regarding  the 
spectra  of  the  red  stars.  However,  the  fourth-type  stars  are 
so  faint  that  only  the  bands  in  their  spectra  could  be  seen 
with  the  telescopes  used  by  these  investigators,  and  their 
numerous  dark  lines  were  beyond  observation.  The  great 
light-grasping  power  of  the  Yerkes  telescope  rendered  a 
photographic  study  of  these  spectra  possible,  with  the  results 
shown  in  Plates  LXXXIII,  LXXXIV,  and  LXXXV.  When 
the  fourth-type  stars  are  ranged  in  a  series,  the  gradual 
change  of  spectrum  from  star  to  star  is  well  illustrated  (Fig.  2, 
Plate  LXXXIII).  The  carbon  flutings  become  stronger  and 
stronger,  until  in  a  star  like  152  Schjellerup  they  are  so  dense 
that  they  cut  out  a  considerable  portion  of  the  light.  In  the 
Sun,  the  Yerkes  telescope  shows  the  existence  of  a  very  thin 
layer  of  carbon  vapor,  lying  in  close  contact  with  the  photo- 
sphere. In  the  fourth-type  stars  we  may  suppose  that  the 
further  process  of  condensation  results  in  an  increased 
development  of  carbon  vapor,  the  absorption  of  which  becomes 
the  characteristic  feature  of  the  spectrum. 

Another  important  point  brought  out  by  this  investigation 
is  the  close  relationship  existing  between  the  line  spectra  of 
third-  and  fourth-type  stars.  As  will  be  seen  by  an  exami- 
nation of  Plate  LXXXV,  the  line  spectra  of  //.  Geminorum 
and  74  Schjellerup  seem  to  be  almost  precisely  identical  in 
certain  regions.  The  presence  of  titanium  oxide  bands  in 
the  one  case,  and  the  carbon  flutings  in  the  other,  compli- 
cate the  comparison  of  the  line  spectra  in  other  regions, 
though  much  is  yet  to  be  learned  on  this  subject  through 
further  study  of  these  stars  with  spectrographs  of  the  highest 
dispersion. 

In  view  of  the  resemblance  of  the  line  spectra,  it  is  diffi- 
cult to  understand  the  diversity  of  the  band  spectra  in  the 
two  great  classes  of  red  stars.  Among  third-type  stars  all 


STELLAR  DEVELOPMENT  197 

intermediate  types  of  spectra  may  be  found  between  the  Sun 
and  the  most  advanced  representative  of  the  class.  It  might 
thus  seem,  especially  in  view  of  the  close  relationship 
between  sun-spot  and  third-type  spectra,  that  the  cooling  of 
the  Sun  would  result  in  the  formation  of  a  third-type  star. 
However,  although  no  such  perfect  continuity  has  been  shown 
to  exist  in  the  transition  from  solar  to  fourth-type  stars,  it 
seems  possible  that  stars  intermediate  in  character  between 
280  Schjellerup  (see  Plate  LXXXV)  and  the  Sun  may  yet  be 
discovered.  Should  this  prove  to  be  the  case,  and  a  more 
rigorous  test  be  found  to  confirm  the  observed  resemblance 
of  the  line  spectra  of  the  third  and  fourth  types,  the  ques- 
tion whether  a  star  like  the  Sun  will  develop  into  a  third-  or 
into  a  fourth-type  star  would  be  difficult  to  answer.  This 
problem,  which  is  one  of  the  most  interesting  of  those  con- 
nected with  the  study  of  stellar  evolution,  will  occupy  a 
prominent  place  in  the  working  programme  of  the  Solar 
Observatory. 

Although  the  red  stars  represent  the  last  period  of  lumi- 
nous stellar  life,  there  remain  to  be  considered  the  dark  stars, 
which  have  been  discovered  in  spite  of  their  complete  invisi- 
bility. Hundreds  of  these  objects  are  already  known  to  us 
through  spectroscopic  observations.  They  are  members  of 
double  or  triple  systems,  moving  in  orbits  about  a  common 
center  of  gravity.  Their  existence  has  been  inferred  from 
measurements  of  the  oscillation  of  the  spectral  lines,  which 
move  back  and  forth  toward  the  red  or  toward  the  violet,  as 
the  star  under  observation  recedes  from  and  then  approaches 
the  earth  in  its  orbital  motion.  Obviously,  it  is  the  spectrum 
of  the  visible  star  which  can  be  observed,  but  motion  in  an 
orbit  necessarily  implies  the  existence  of  a  companion  star, 
which  may  or  may  not  be  luminous.  If  sufficiently  bright 
to  be  visible,  it  may  not  be  separated  from  its  close  rieighbor 
in  the  most  powerful  telescopes.  But  in  the  spectroscope 


198  STELLAR  EVOLUTION 

the  lines  of  the  composite  spectrum  will  appear  double,  twice 
in  each  orbital  revolution  of  the  pair.  If  only  one  star  is 
bright  enough  to  give  a  spectrum,  its  lines  will  simply  oscil- 
late to  and  fro. 

Hitherto  we  have  tacitly  assumed,  in  harmony  with  cur- 
rent views,  that  all  stars  are  built  on  a  single  model,  and  that 
each  passes  through  the  same  stages  of  development  in  its 
transition  from  the  nebular  condition  to  the  solid  state.  It 
should  be  pointed  out  here,  however,  that  many  circumstances 
warn  us  against  implicit  acceptance  of  such  a  law  of  uni- 
formity. The  assumption  that  a  given  type  of  spectrum 
represents  a  given  stage  of  growth  involves  the  idea  that  the 
chemical  composition  of  all  stars  is  essentially  the  same,  and 
that  the  particular  position  of  the  star  in  the  universe,  and 
other  conditions  which  may  obtain  in  individual  cases,  are 
matters  of  no  importance.  While  it  is  true  that  we  have 
strong  reasons  for  belief  in  the  universal  distribution  of  most 
of  the  chemical  elements  known  on  the  Earth,  and  the  uni- 
versal operation  of  the  law  of  gravitation,  and  of  all  other 
laws  which  define  terrestrial  conditions,  the  assumption  that 
identically  the  same  course  is  pursued  by  every  star  in 
passing  from  its  origin  to  its  final  decay  is  entirely  un- 
warranted. We  must  be  prepared  to  meet  widely  diverse 
conditions  and  to  observe  modifications  in  the  process  of 
development  which  are  determined  directly  by  such  con- 
ditions. 

Take  the  case  of  the  Pleiades,  for  example  (Plate 
LXXXVI).  Here  we  have  a  group  of  stars  entangled  in 
nebulosity,  and  moving  together  through  the  heavens. 
Every  indication  goes  to  show  that  this  is  an  organic  group, 
whose  members  are  of  common  origin.  But  the  spectra  of 
practically  all  of  these  stars,  irrespective  of  size  and  bright- 
ness, are  of  Secchi's  first  type.  How  are  we  to  believe  that 
widely  different  masses  will  pass  through  their  evolutional 


STELLAR  DEVELOPMENT  199 

steps  with  equal  rapidity  ?  An  appeal  to  double  stars,  whose 
members  are  undoubtedly  of  common  origin,  does  not  help 
matters.  We  invariably  find  that  the  fainter  member  of  the 
system,  which  might  have  been  supposed  to  cool  most  rapidly 
because  of  its  smaller  size,  is  yellow  or  red  in  color,  while  its 
larger  companion  is  more  nearly  white,  or  tinged  with  blue. 
Huggins  argues,  however,  that  the  greater  surface  gravity 
possessed  by  stars  of  large  mass  may  cause  more  rapid  change 
in  spectral  type.  Thus  a  large  star  of  low  density  may  be  no 
farther  advanced  in  spectral  type  than  a  smaller  but  more 
highly  condensed  star.  According  to  Huggins'  views,  the 
early  steps  in  evolution  would  be  characterized  by  small 
gravity  at  the  surface,  comparatively  slow  changes  of  tempera- 
ture in  passing  outward  from  the  interior,  and  convection 
currents  less  violent  than  those  observed  in  the  Sun.  If  the 
star  were  hot  enough,  hydrogen  might  be  the  only  gas  suf- 
ficiently cool,  with  respect  to  the  radiation  from  below,  to 
show  itself  by  absorption  lines.  Vapors  of  greater  density 
would  lie  lower  in  the  star,  where  their  temperature  might 
be  so  nearly  that  of  the  region  behind  them  that  their  lines 
would  not  appear  in  the  spectrum. 

Schuster,  who  has  done  an  important  service  in  empha- 
sizing the  elements  of  weakness  in  the  assumed  law  of  uni- 
formity, nevertheless  believes  that  most  of  the  spectral  types 
represent  stages  in  the  development  of  stars.  Thus,  while 
he  does  not  maintain  that  all  stars  pass  through  an  identical 
series  of  changes,  he  agrees  with  the  view  that  the  general 
course  of  development  lies  along  similar  lines,  though  impor- 
tant modifications  may  enter  in  particular  cases.  The  order 
of  development  which  he  favors  is  as  follows : 

(1)  Helium  or  Orion  stars. 

(2)  Hydrogen  or  Sirian  stars. 

(3)  Calcium  or  Procyon  stars. 

(4)  Solar  or  Capellan  stars. 


200  STELLAE  EVOLUTION 

In  describing  the  process  of  condensation,  Schuster  points 
out  that  the  expansion  caused  by  the  rising  temperature  of 
the  gaseous  bodies  must  at  first  result  in  the  rejection  of 
heliiim,  hydrogen,  and  other  light  gases,  on  the  supposition 
that  the  gravitation  is  not  sufficient  to  retain  them.  These 
light  gases  will  thus  be  left  to  constitute  diffuse  nebulous 
masses,  as  illustrated  by  the  gaseous  nebulae,  particularly 
by  the  nebulous  regions  in  such  a  group  as  the  Pleiades. 
In  the  process  of  time,  however,  the  star  will  have  condensed 
sufficiently  to  retain  hydrogen  and  helium,  and  these  gases 
will  then  begin  to  diffuse  into  the  interior,  where  they  will 
be  absorbed  at  a  rate  which  depends  upon  the  star's  mass. 
Helium,  which  is  denser  than  hydrogen,  will  be  retained 
first,  thus  giving  rise  to  the  helium,  or  Orion,  stars.  As  this 
gas  diffuses  inward,  its  place  will  be  taken  by  hydrogen, 
which  will  thus  become  predominant  in  the  spectrum.  In 
its  turn,  the  hydrogen  will  diffuse  into  the  star,  and  the 
increasing  convection  currents  will  cause  a  more  and  more 
complete  stirring-up  of  the  low-lying  metallic  vapors,  which 
will  therefore  play  an  increasingly  prominent  part  in  the 
spectrum.  Thus  the  solar  stage  will  ultimately  be  reached. 

An  interesting  point  in  this  explanation  is  the  consider- 
able possibility  of  variation  which  stars  of  different  mass  and 
in  different  environments  may  exhibit.  If  but  little  hydrogen 
happens  to  be  in  the  neighborhood,  the  process  of  conden- 
sation may  not  result  in  the  attraction  of  a  sufficient 
quantity  of  this  gas  to  produce  the  hydrogen  type  of  spec- 
trum. Again,  the  star  may  be  of  such  low  density  that  it  is 
unable  to  attract  hydrogen,  and  thus  it  may  pass  into  the 
solar  stage  without  exhibiting  strong  hydrogen  lines.  There 
may  also  be  stars  of  such  small  mass  that,  in  spite  of  having 
condensed  sufficiently  to  attract  hydrogen,  they  are  not  able 
to  absorb  it  all,  and  therefore  they  may  continue  to  exhibit 
a  spectrum  of  the  first  type  without  ever  passing  into  the 


STELLAR  DEVELOPMENT  201 

solar  stage.  Furthermore,  in  the  case  of  two  stars  of  equal 
age  but  different  mass,  the  larger  may  have  passed  to  the 
condition  of  Arcturus  (incipient  red  star),  while  the  other  is 
still  in  the  solar  stage,  because  of  its  more  rapid  absorption 
of  hydrogen. 

This  theory  gives  an  interesting  explanation  of  the  above- 
mentioned  spectral  phenomena  of  double  stars,  since  it  indi- 
cates that  the  larger  star,  through  its  power  of  absorbing 
hydrogen  more  rapidly  and  completely,  may  pass  to  the  solar 
stage,  while  the  smaller  one  continues  to  give  a  spectrum  of 
the  first  type.  Schuster  agrees  with  Huggins  that  the  small 
mass  would  lose  heat  more  rapidly  than  the  larger  one,  but 
believes  that  the  type  of  spectrum  may  be  more  completely 
controlled  by  the  rapidity  with  which  the  hydrogen  is 
absorbed. 

It  is  evident  that  the  highly  suggestive  views  of  Schuster 
should  stimulate  much  research.  The  distribution  of  spectra 
of  different  types  through  the  heavens  is  a  subject  of  great 
interest,  and  doubtless  has  an  important  bearing  on  the 
question  of  stellar  evolution.  Certain  types  of  stars,  for 
example,  tend  to  cluster  thickly  in  the  Milky  Way,  while 
others  show  no  such  tendency.  Pickering's  work  in  photo- 
graphing the  spectra  of  an  immense  number  of  stars  in  the 
northern  and  southern  heavens  offers  most  valuable  material 
for  the  study  of  this  subject.  The  investigation  may  perhaps 
be  extended  to  fainter  stars  with  the  60-inch  Mount  Wilson 
reflector,  through  the  use  of  a  spectrograph  having  no  slit 
and  so  designed  as  to  record  photographically  the  spectra  of 
all  stars  lying  within  a  certain  field.  But  since  this  field 
can  hardly  exceed  20'  of  arc  in  diameter,  it  would  not  be 
feasible  to  photograph  the  entire  heavens  in  this  way.1 

However,  a  most  important  scheme  of  co-operation  has 
been  instituted  by  Kapteyn,  for  the  purpose  of  obtaining 

1  The  objective  prism  photographs  cover  a  field  several  degrees  in  diameter. 


202  STELLAR  EVOLUTION 

data  bearing  upon  the  problem  of  the  geometrical  stucture 
of  the  universe  and  the  distribution  of  stars  within  it. 
Through  the  impracticability  of  securing  all  necessary  data 
for  stars  distributed  over  the  entire  heavens,  Kapteyn  has 
selected  certain  limited  areas  of  the  sky,  so  distributed  as  to 
render  it  probable  that  conclusions  based  upon  a  complete 
study  of  the  stars  within  these  areas  will  be  likely  to  apply 
to  the  heavens  at  large.  The  application  of  the  60-inch 
reflector  to  the  photography  of  stellar  spectra  by  the  above- 
mentioned  process  will  therefore  be  confined,  for  the  most 
part,  to  Kapteyn's  areas,  where  many  other  observers  are 
already  gathering  information,  in  accordance  with  a  plan 
which  allots  to  each  institution  the  work  for  which  its  instru- 
ments are  best  adapted.  For  Kapteyn's  purposes,  only  the 
general  type  of  spectrum  is  required,  since  he  is  primarily 
concerned  with  questions  of  distribution  and  structure,  rather 
than  those  which  relate  to  the  evolution  of  stars.  The  data  he 
desires  include  determinations  of  the  brightness,  distance, 
and  motions  of  the  stars  within  the  selected  areas.  The  60- 
inch  reflector,  on  account  of  its  great  light-gathering  power, 
can  assist  materially  in  those  portions  of  this  work  which 
relate  to  the  faintest  stars.  The  investigation  of  the  motions 
of  these  stars  in  the  line  of  sight  is  necessary  from  the  evo- 
lutional standpoint,  because  community  of  motion  may  mean 
organic  relationship  of  stars  in  a  group,  as  in  the  case  of 
the  Pleiades.  Photometric  investigations  and  the  study  of 
parallaxes  are  also  required,  since,  when  the  distance  and 
brightness  of  a  star  are  known,  its  mass  can  be  determined, 
if  certain  reasonable  assumptions  as  to  the  surface  brilliancy 
are  made.  We  have  just  seen  how  important  a  factor  the 
mass  of  a  star  may  be  in  determining  the  course  of  its 
evolution. 

Enough  has  been  said  to  indicate  the  nature  of  the  work 
which  large  telescopes  may  perform.     The  direct  photography 


STELLAE  DEVELOPMENT  203 

of  nebulae  may  provide  the  means  of  detecting,  in  the  course 
of  years,  changes  in  their  form  bearing  directly  upon  the 
manner  and  rate  of  their  condensation.  The  photographic 
study  of  their  spectra  may  help  to  explain  why  a  few  nebulae 
show  the  bright  line  spectra  of  gases,  while  the  very  numerous 
spiral  nebulae  appear  to  have  merely  a  continuous  spectrum. 
With  the  high  dispersion  of  powerful  spectrographs  like  the  one 
shown  in  the  constant-temperature  chamber  in  Plate  XCVI, 
the  spectra  of  a  few  of  the  brightest  stars  in  the  heavens, 
which  include  most  of  the  spectral  types,  can  be  minutely 
analyzed.  In  this  way,  and  with  the  aid  of  smaller  spectro- 
graphs, the  spectra  of  red  stars  of  the  third  and  fourth  types 
can  be  examined  to  much  better  advantage  than  previously, 
with  reference  to  their  relationship  to  the  Sun,  to  sun-spots, 
and  to  one  another.  It  is  evident  that  these  investigations, 
with  others  on  the  spectra  of  stars  of  various  classes,  the 
distribution  of  the  different  types  of  spectra  within  Kapteyn's 
selected  areas,  and  studies  of  the  brightness  and  parallaxes 
of  the  same  stars,  might  well  involve  the  co-operative  use  of 
several  telescopes  of  the  largest  size. 


CHAPTER   XXI 
THE  METEORITIC  AND  PLANETESIMAL    HYPOTHESES 

IN  even  the  briefest  outline  of  the  methods  of  studying 
stellar  evolution,  reference  must  be  made  to  two  hypotheses 
which  are  intended  by  their  authors  to  take  the  place  of  the 
nebular  hypothesis  of  Laplace.  In  both  of  these,  swarms 
of  meteorites,  rather  than  matter  in  the  gaseous  state,  are 
supposed  to  afford  the  raw  material  of  which  stellar  systems 
are  compounded.  The  nature  of  the  swarms,  however,  is 
unlike  in  the  two  cases.  According  to  Lockyer,  the  meteorites 
are  to  be  regarded  as  analogous  to  the  wandering  mole- 
cules of  gases,  in  that  they  move  indiscriminately  in  all  direc- 
tions and  at  widely  different  velocities.  Sir  George  Darwin 
has,  indeed,  demonstrated  mathematically  that  a  meteoritic 
swarm,  constituted  in  this  way,  is  closely  analogous  to  a 
gas.  The  meteorites  move  rapidly  about,  colliding  with  one 
another  from  time  to  time,  just  as  the  molecules  of  a  gas  are 
supposed  to  do,  according  to  the  kinetic  theory.  Chamberlin 
and  Moulton,  on  the  contrary,  assume  their  meteorites  to  be 
revolving  in  well-defined  orbits,  and  therefore  suffering  only 
such  collisions  as  may  result  from  certain  meteorites  over- 
taking others  of  lower  velocity.1 

The  most  characteristic  nebular  line  is  a  brilliant  one  in 
the  green  part  of  the  spectrum,  attributed  to  an  unknown 
gas,  which  has  been  called  "nebulum."  According  to 
Lockyer,  this  line  is  the  remnant  of  a  complicated  fluting 
in  the  spectrum  of  magnesium  oxide,  with  the  brightest 
part  of  which  he  found  it  exactly  to  coincide.  In  his  view 

t      1  In  his  explanation  of  globular  and  spiral  nebulae,  and  of  certain  other  celestial 
phenomena,  Lockyer  also  assumes  the  meteorites  to  revolve  in  well-defined  orbits. 

204 


METEORITIC  AND  PLANETESIMAL  HYPOTHESES    205 

the  rest  of  the  fluting  is  invisible  only  because  of  the  faint- 
ness  of  the  nebular  line.  This  has  been  completely  dis- 
proved, however,  by  Keeler's  remarkably  precise  measures 
of  the  chief  nebular  line  at  the  Lick  Observatory.  His 
observations  show,  not  only  that  the  chief  nebular  line  does 
not  correspond  in  position  with  the  head  of  the  magnesium 
fluting,  but  also  that  it  differs  entirely  from  it  in  appearance. 
It  is  therefore  not  possible  to  regard  magnesium  oxide  as  a 
constituent  of  the  nebulae.  The  green  line  may  with  far 
greater  probability  be  considered  to  represent  a  very  light 
gas,  not  yet  discovered  on  the  Earth. 

Lockyer's  conclusions  as  to  the  origin  of  the  chief  nebular 
line  play  an  important  part  in  his  meteoritic  hypothesis. 
He  believes  that  the  frequent  collisions  between  meteorites 
in  the  swarms  produce  sufficient  heat  to  volatilize  certain 
constituents  of  the  meteorites,  which  are  rendered  luminous, 
so  that  their  lines  should  appear  in  the  nebular  spectrum. 
Lockyer  tried  the  experiment  of  heating  fragments  of  mete- 
orites in  a  tube,  from  which  the  air  had  been  partially 
exhausted.  He  found  that  hydrogen,  hydrocarbon  vapors, 
and  the  vapor  of  magnesium  oxide  were  given  off  from  the 
meteorites.  When  an  electric  discharge  was  passed  through 
the  gases  in  the  tube  at  reduced  pressure,  the  spectrum  was 
found  to  consist  of  the  lines  of  hydrogen,  the  characteristic 
flutings  given  by  compounds  of  carbon,  and  the  green  fluting 
of  magnesium  oxide,  to  which  reference  has  been  made.  In 
comets,  which  are  known  to  be  intimately  associated  with 
meteorites,  the  flutings  due  to  compounds  of  carbon  form 
the  most  characteristic  feature  of  the  spectrum.  But  although 
certain  astronomers  have  believed  these  flutings  to  be  present 
in  the  spectrum  of  the  nebulae,  their  conclusions  are  not 
confirmed  by  the  majority  of  observers,  who  can  neither 
see  nor  photograph  any  trace  of  the  flutings.  The  only 
remaining  connection  between  the  nebulae  and  the  gases 


206  STELLAR  EVOLUTION 

derived  by  Lockyer  from  meteorites  therefore  depends  upon 
the  presence  of  hydrogen  in  both  cases.  But  hydrogen  is 
so  universally  distributed  among  the  celestial  bodies  that  its 
absence  from  nebulae  would  almost  be  regarded  as  an  anom- 
aly requiring  explanation.  It  therefore  cannot  be  said  that 
much  weight  is  to  be  accorded  to  the  experimental  basis  of 
the  meteoritic  hypothesis. 

It  ought  to  be  said,  in  favor  of  the  hypothesis,  that  it 
provides  a  simple  way  of  accounting  for  the  existence  in 
the  nebulae  of  substances  not  represented  in  their  spectra, 
but  which  appear  in  stars  evolved  from  nebulae.  If  a 
nebula  is  to  be  regarded  as  a  glowing  gas,  in  which  all 
substances  contained  in  stars  exist  in  a  state  of  vapor, 
it  remains  to  be  shown  why  a  very  few  gases  manifest 
their  presence  by  the  appearance  of  their  bright  lines  in 
the  spectrum,  whereas  all  the  other  elements  produce  no 
lines,  and  therefore  give  no  indication  of  their  existence. 
In  this  connection  it  must  not  be  forgotten  that  in  mix- 
tures of  various  vapors  the  spectra  of  some  of  the  vapors 
appear  when  an  electric  discharge  is  passed  through  the 
mixture,  while  the  lines  due  to  certain  other  vapors  remain 
invisible.  Too  little  has  been  done,  however,  in  this  im- 
portant field  of  research,  to  permit  final  conclusions  to  be 
drawn.  For  this  reason  no  one  is  at  present  able  to  say  in 
what  form  the  iron,  nickel,  and  other  metals,  which  sub- 
sequently make  their  appearance  in  the  stars,  can  exist  in 
the  nebulae. 

This  question  is,  indeed,  but  one  of  the  many  mysteries 
which  at  present  surround  the  nebulae  (Plates  LXXXVI- 
XC).  We  have  no  knowledge,  for  example,  why  they  glow 
with  a  steady  and  unchanging  light,  since  there  is  no  direct 
evidence  that  this  light,  is  produced  either  by  heat  or  by 
electrical  excitation.  It  must  not  be  forgotten  that  very  few 
nebulae  are  certainly  known  to  be  gaseous:  thousands  of 


METEORITIC  AND  PLANETESIMAL  HYPOTHESES    207 

them  seem  to  give  a  continuous  spectrum,  in  which  the  bright 
lines  of  gases  do  not  appear.  Whether  this  is  due  to  the 
presence  of  solid  or  liquid  matter,  to  pressure  effects,  or  to 
other  causes,  is  not  yet  known.  The  process  by  which  stars 
are  condensed  out  of  nebulae  is  also  not  clearly  understood. 
It  cannot  depend  wholly  upon  some  action  connected  with  the 
spiral  form,  since,  as  already  stated,  we  have  in  the  Orion 
nebula,  which  is  not  a  spiral,  one  of  the  best-known  examples 
of  direct  relationship  between  stars  and  nebulae.  It  is  now 
rather  commonly  believed  that,  while  the  temperature  of 
small  particles  in  the  nebulae  may  be  very  high,  the  mean 
temperature  of  the  entire  mass  may  nevertheless  be  very  low, 
since  it  has  been  pointed  out  by  Huggins  that  the  appear- 
ance presented  by  the  nebulae  could  be  produced  by  widely 
separated  luminous  particles.  In  view  of  all  these  facts,  it 
may  therefore  be  said  that  much  work  remains  to  be  done 
on  the  nebulae,  not  only  in  photographing  their  forms,  but  in 
investigating  their  spectra,  and  in  interpreting  them  through 
laboratory  experiments. 

Starting  from  the  meteoritic  hypothesis,  and  assuming 
that  the  chemical  elements,  at  the  temperature  of  the  hottest 
stars,  are  dissociated  into  simpler  substances,  Lockyer  has 
developed  a  plan  of  stellar  evolution  which  comprises  a  classi- 
fication of  stellar  spectra  on  a  temperature  basis.  He  sup- 
poses that  the  meteoritic  swarms  represented  by  the  nebulae 
gradually  condense  into  stars,  by  processes  whose  details  are 
still  uncertain.  According  to  his  classification,  the  gaseous 
and  bright-line  stars,  in  which  the  temperature  is  supposed 
to  be  higher  than  that  of  the  less  condensed  nebulae,  lie  just 
above  the  latter  in  point  of  development.  Then  come  the 
red  stars  of  Secchi's  third  type :  though  it  may  appear  to  many 
spectroscopists  that  the  difficulty  of  tracing  a  connection 
between  their  spectra  and  those  of  the  stars  placed  just  before 
them  would  be  altogether  insuperable.  Further  condenr 


208  STELLAR  EVOLUTION 

sation,  still  involving  a  rise  of  temperature,  would  produce 
stars  analogous  to  the  Sun,  but  differing  in  the  important 
particular  that,  while  their  temperature  is  increasing,  that 
of  the  Sun  is  supposed  to  be  decreasing.  Finally,  at  the 
point  of  maximum  temperature,  Lockyer  places  stars  of 
Secchi's  first  type.  Here  the  meteorites,  long  since  com- 
pletely transformed  into  the  gaseous  state,  have  reached  the 
condition  implied  by  Lane's  law,  at  which  the  rise  in  super- 
ficial temperature,  due  to  continued  condensation,  is  just 
balanced  by  the  loss  resulting  from  radiation.  The  declining 
period,  then  setting  in,  results  in  the  development  of  stars 
like  the  Sun,  which  can  be  only  arbitrarily  distinguished  from 
stars  of  equal,  but  rising,  temperature,  lying  on  the  opposite 
branch  of  the  temperature  curve.  After  the  solar  stars  come 
the  red  stars  of  Secchi's  fourth  type,  and  after  these,  final 
extinction  of  light. 

This  system  of  classification,  considered  apart  from  the 
hypotheses  with  which  it  is  connected,  has  the  advantage  of 
providing  for  both  the  ascending  and  descending  branches 
of  the  temperature  curve.  Unfortunately,  we  are  perhaps  not 
yet  in  a  position  to  distinguish  clearly  between  stars  of  the 
same  surface  temperature,  in  one  of  which  the  gain  of  heat 
is  more  rapid  than  the  loss,  while  in  the  other  the  reverse 
is  true.  As  already  remarked,  the  assumption  that  the  red 
stars  of  Secchi's  third  type  lie  not  far  above  the  nebulae  is  also 
a  difficult  one  to  admit.  But  the  classification  nevertheless 
deserves  careful  consideration,  and  the  most  searching  tests 
that  can  be  applied. 

As  the  late  Miss  Clerke  has  well  said,  the  complex  struc- 
ture of  meteorites  suggests  a  highly  developed,  rather  than  an 
elementary,  condition  of  existence.  This,  however,  is  hardly 
to  be  taken  as  an  objection  to  Lockyer' s  hypothesis,  since 
the  manner  in  which  the  meteoritic  swarms  came  into  exist- 
ence is  not  postulated.  The  planetesimal  hypothesis, however, 


METEORITIC  AND  PLANETESIMAL  HYPOTHESES    209 

begins  with  a  fully  organized  sun,  which  is  supposed,  in  its 
motion  through  space,  to  come  into  the  immediate  neighbor- 
hood of  another  sun,  equal  to  or  greater  than  itself.  The 
effect  of  the  attraction  between  the  two  bodies  would  be  to 
reduce  the  immense  restraining  power  of  the  Sun  along  the 
line  of  mutual  attraction,  i.  e.,  in  the  direction  of  the  other 
sun,  and  in  the  opposite  direction.  Under  certain  conditions 
the  Sun  is  observed  to  shoot  out  prominences  with  velocities 
approaching  300  miles  per  second.  If  the  velocity  exceeded 
382  miles  per  second,  the  matter  projected  from  the  Sun 
would  escape  the  power  of  its  attraction  and  move  off  into 
space,  never  to  return.  If  another  great  body  were  passing 
near  the  Sun,  the  tendency  toward  eruptions  would  be  greatly 
augmented  along  the  line  joining  the  two  bodies,  and  immense 
protuberances  would  doubtless  be  projected  at  high  velocities 
from  opposite  ends  of  the  solar  diameter  corresponding 
with  this  line. 

According  to  the  planetesimal  hypothesis,  the  two  pro- 
tuberances would  be  formed  as  the  two  suns  were  swinging 
past  one  another  around  their  common  center  of  gravity. 
The  effect  of  mutual  attraction  would  be  to  cause  the  two 
great  arms  to  assume  a  spiral  form,  in  which  the  scattered 
materials  revolve  about  the  central  mass  in  elliptical  orbits. 
Moulton  has  shown,  by  rigorous  mathematical  tests,  that  just 
such  a  result  might  actually  occur,  and  that  the  forms  of 
the  spiral  nebulae  may  thus  be  closely  imitated  (Plates 
LXXXVIII-XC).  Although  the  matter  shot  out  from  the 
Sun  would  necessarily  be  gaseous,  the  hypothesis  assumes 
that  it  wo  aid  rapidly  cool  down  to  a  finely  divided  solid  con- 
dition.1 The  outer  portions  of  the  protuberances  would 
naturally  be  formed  from  the  surface  materials  of  the  Sun, 
while  the  inner  extremities  would  come  mainly  from  lower 

i  How,  it  may  be  asked,  can  these  small  bodies  remain  brilliantly  luminous  for 
many  years?  And  why  do  we  not  discover  incipient  spirals,  giving  a  bright  line 
spectrum? 


210  STELLAR  EVOLUTION 

depths,  where  the  heavier  elements  are  found.  This  may 
possibly  explain  the  lightness  of  the  outer  planets  of  our  solar 
system,  and  the  great  relative  weight  of  the  inner  ones.  The 
changing  attraction  of  the  neighboring  star  might  also  cause 
a  series  of  irregular  outbursts,  accounting  for  the  knotty  and 
uneven  distribution  of  the  matter  in  the  spirals  (Plate  XC). 
Chamberlin  points  out  that  a  very  small  fraction  of  the  Sun's 
mass,  not  exceeding  1  or  2  per  cent. ,  would  be  amply  sufficient 
to  supply  all  of  the  matter  required  to  form  a  planetary 
system  like  our  own. 

In  the  further  evolution  of  the  system,  the  central  mass 
is  supposed  to  form  the  sun,  the  knots  to  serve  as  the  nuclei 
about  which  the  planetary  materials  gather,  and  the  remain- 
ing diffuse  nebulous  matter  to  be  swept  up  by  the  nuclei  or 
absorbed  by  the  sun.  The  building-up  of  the  planets  is  not 
supposed  to  take  place,  as  in  the  nebular  hypothesis,  simply 
through  the  gravitational  attraction  of  the  planetary  nuclei 
on  the  matter  surrounding  them.  On  the  contrary,  the  main 
agency  is  assumed  to  be  a  gradual  accretion  of  the  mass 
through  collisions  of  isolated  planetesimals  (meteorites) 
resulting  from  the  intersection  of  the  individual  orbits, 
brought  about  periodically  through  the  rotation  of  their  line 
of  apsides.  Thus  it  is  held,  according  to  this  hypothesis, 
that  the  Earth  was  never  a  molten  mass,  but  that  it  was  built 
up  by  gradual  accretions.  Chamberlin  was  led  to  this  view 
of  the  condition  of  the  Earth's  interior  from  various  geo- 
logical considerations,  which  seem  to  him  inconsistent  with 
the  hypothesis  of  a  fluid  origin. 

If  this  book  were  a  treatise  on  stellar  evolution,  all  of 
these  questions  would  require  much  fuller  discussion  and 
criticism,  and  space  would  necessarily  be  devoted  to  the 
remarkable  phenomena  of  variable  and  temporary  stars,  the 
tidal  investigations  of  Darwin  and  their  possible  bearing  on 
the  evolution  of  double-star  systems,  and  many  other  subjects 


METEOKITIC  AND  PLANETESIMAL  HYPOTHESES    211 

which  have  not  received  consideration.  Enough  has  been 
said,  however,  to  give  an  idea  of  the  nature  of  the  problems 
which  an  observer  concerned  with  stellar  evolution  is  called 
upon  to  attack,  and  the  general  character  of  some  of  the 
observational  methods  required  to  solve  them. 


CHAPTER  XXII 
DOES  THE  SOLAR  HEAT  VARY  ? 

ONE  does  not  often  stop  to  think  of  the  delicate  balance 
that  determines  the  conditions  of  life  on  the  Earth.  But  it 
is  obvious  enough  that  a  small  change  in  the  intensity  of  the 
solar  radiation  would  suffice  to  transform  the  climate  of  the 
temperate  zones  to  that  of  the  equatorial  or  polar  regions. 
A  greater  change  might  soon  result  in  the  complete  destruc- 
tion of  life. 

It  is  therefore  a  matter  of  the  most  vital  interest  to 
inquire  into  the  source  and  constancy  of  the  Sun's  heat. 
What  fuel  maintains  the  great  fire  that  warms  and  lights  .us, 
and  supplies,  through  its  beneficent  influence  on  growing 
crops,  the  food  that  we  consume?  Is  the  average  daily 
influx  of  solar  rays  constant  and  unchangeable,  and  are  we 
justified  in  our  tacit  belief  in  the  inexhaustibility  of  the 
supply?  Such  thoughts,  seriously  pondered  by  students  of 
solar  physics,  have  led  to  extensive  investigations,  which 
must  go  on  for  many  years  before  these  questions  can  be 
finally  answered. 

As  we  have  already  seen,  the  contraction  of  a  nebulous 
mass  to  form  a  star,  or  a  sun  like  our  own,  must  result  in 
the  liberation  of  much  heat.  Indeed,  the  total  solar  radia- 
tion in  the  course  of  a  year  can  be  accounted  for  on  the  sup- 
position that  the  Sun's  diameter  decreases  about  250  feet  in 
this  time.  Since  the  discovery  of  radium,  which  possesses 
the  remarkable  property  of  sending  out  heat,  with  little  evi- 
dence of  exhaustion,  for  very  long  periods  of  time,  it  has 
been  suggested  that  this  substance,  if  it  exists  in  the  Sun, 
may  be  i/he  source  of  part  of  its  radiation.  Radium  has  not 

212 


DOES  THE  SOLAR  HEAT  VARY?  213 

yet  been  detected  in  the  Sun  with  the  spectroscope,  but  it 
may  lie  at  low  levels,  where  its  vapor  would  take  no  part  in 
the  absorption  that  produces  the  lines  of  the  solar  spectrum. 
The  abundance  of  helium  in  the  Sun  suggests  that  radium, 
which  gives  off  this  gas  during  the  disintegration  process, 
may  perhaps  exist  within  or  beneath  the  photosphere. 

If  radium  really  supplies  any  considerable  part  of  the 
Sun's  heat,  its  ultimate  exhaustion  would  involve  a  decided 
decrease  in  the  solar  radiation.  As  we  are  not  yet  certain, 
however,  that  there  is  any  radium  in  the  Sun,  the  possibility 
of  such  a  contingency  may  be  regarded  as  too  remote  for 
profitable  speculation. 

We  may  take  it  for  granted  that  the  Sun  will  continue  to 
radiate  heat,  at  practically  the  present  average  rate,  for  many 
centuries  to  come.  But  do  we  know  that  the  rate  is  abso- 
lutely constant?  May  not  fluctuations  occur  of  sufficient 
magnitude  to  affect  our  climate  appreciably,  and  to  be 
reflected  in  the  ebb  and  flow  of  crops  and  the  price  of  wheat  ? 

Until  a  short  time  ago  this  question  had  been  tested  in 
only  the  roughest  way.  It  was  known  that  sun-spots  pass 
through  a  regular  cycle  of  change,  occupying  about  eleven 
years.  A  curve  was  accordingly  drawn,  showing  the  varying 
number  of  sun-spots,  and  compared  with  a  curve  represent- 
ing, for  example,  the  varying  price  of  wheat.  As  the  two 
were  thought  to  show  some  correspondence  in  form,  it  was 
held  that  the  price  of  wheat  is  determined  by  the  solar 
activity,  as  measured  by  the  number  of  spots. 

But  the  correspondence  of  the  two  curves  was  far  from 
perfect,  and  might  have  resulted  from  mere  chance.  Rain- 
fall and  temperature  curves  have  given  results  that  appear 
more  satisfactory,  but  the  whole  question  is  still  in  its  primi- 
tive stages,  and  little  that  is  absolutely  definite  and  reliable 
has  been  learned.  The  efforts  now  being  made  by  the  Solar 
Commission  of  the  International  Meteorological  Committee 


214  STELLAR  EVOLUTION 

may  be  expected  to  help  matters,  but  much  will  depend  upon 
the  appliances  used  to  measure  the  solar  radiation,  and  to 
determine  the  amount  of  heat  lost  by  absorption  in  the 
Earth's  atmosphere. 

The  most  elaborate  study  of  this  question  yet  made  is 
due  to  the  late  Secretary  Langley,  of  the  Smithsonian  Insti- 
tution. He  long  ago  recognized  that  the  chief  difficulty  of 
the  problem  lies  in  the  constantly  varying  absorption  of  the 
air  above  us.  If  measures  of  the  solar  radiation  could  be 
made  from  a  point  outside  of  our  atmosphere,  any  observed 
fluctuations  would  be  due  to  the  Sun  itself.  But  near  the 
level  of  the  sea  the  difficulties  are  very  great. 

To  diminish  them,  Langley  led  an  expedition  to  Mount 
Whitney  in  California.  Here,  at  an  elevation  of  over  15,000 
feet,  the  denser  and  more  variable  half  of  the  atmosphere  is 
left  below.  The  precision  of  the  measures  was  thus  greatly 
increased,  but  the  expedition  was  not  able  to  remain  long 
enough  to  determine  whether  the  so-called  "solar  constant" 
of  radiation  is  actually  a  constant,  or  undergoes  changes  of 
an  irregular  or  a  periodic  character. 

Langley  strongly  felt  the  importance  of  continuing  this 
work  with  the  greatly  improved  apparatus  developed  by 
Abbot  and  others  at  the  Smithsonian  Astrophysical  Obser- 
vatory in  Washington.  He  therefore  recommended  that  the 
Carnegie  Institution  make  provision  for  further  researches 
of  this  nature  at  a  mountain  station.  When  the  Solar  Ob- 
servatory was  established,  a  co-operative  arrangement  with 
the  Smithsonian  Institution  was  accordingly  entered  into, 
and  measures  of  the  solar  constant  were  made  daily  by  Abbot 
on  Mount  Wilson  during  the  summers  of  1905  and  1906. 

The  apparatus  used  in  this  work  is  most  ingenious.  Two 
independent  operations  are  carried  on  simultaneously:  the 
direct  measurement  of  the  solar  radiation  with  some  form  of 
pyrheliometer ;  and  the  determination  of  the  atmospheric 


DOES  THE  SOLAR  HEAT  VARY?  215 

absorption,  for  all  the  colors  of  the  spectrum,  with  a  bolometer 
( Plate  XCI). 

The  pyrheliometer,  in  the  form  used  by  Abbot,  measures 
the  rise  in  temperature,  in  a  given  time,  of  a  known  volume 
of  liquid  exposed  to  the  Sun's  rays.  If  there  were  no  atmos- 
phere, pyrheliometer  measures  alone  would  suffice  to  furnish 
the  desired  information.  But  the  heat  of  the  Sun  at  noon 
is  far  greater  than  shortly  after  sunrise,  since  the  rays  pass 
through  a  much  shorter  air-path.  Consequently,  the  obser- 
vations must  be  repeated  at  regular  intervals  throughout  the 
morning. 

The  bolometer,  invented  by  Langley,  is  so  sensitive 
to  radiation  that  it  will  measure  a  rise  in  temperature  of 
less  than  one-millionth  of  a  degree.  It  consists  of  two 
very  fine  threads  of  platinum,  about  -g-yVir  inch  thick, 
mounted  side  by  side  within  a  constant  temperature  chamber. 
One  of  these  is  shielded,  the  other  exposed  to  the  radiation 
to  be  measured.  The  platinum  threads  form  two  of  the  arms 
of  a  "Wheatstone's  bridge,"  and  are  connected  with  a  stor- 
age battery,  so  that  a  feeble  current  constantly  passes  through 
them.  A  galvanometer  of  the  most  sensitive  type  is  so  bal- 
anced in  the  circuit  that  its  reading  is  zero  when  the  currents 
flowing  through  the  two  platinum  threads  are  equal.  The 
moment  the  resistance  of  the  exposed  strip  is  changed  by 
radiation  falling  upon  it,  the  galvanometer  is  deflected  by 
an  amount  which  measures  the  heating  effect  of  the  radiation. 

In  practice,  the  solar  spectrum  is  caused  to  move  slowly 
across  the  exposed  bolometer  thread.  The  galvanometer 
needle  then  swings  back  and  forth,  giving  small  deflections 
when  a  dark  line  or  absorption  band  is  passing  over  the 
bolometer,  and  large  deflections  when  the  full  intensity  of 
the  spectrum  is  being  measured.  To  record  the  motions  of 
the  needle  a  minute  mirror,  attached  to  it,  is  caused  to  reflect 
a  spot  of  light  upon  a  photographic  plate.  The  same  mechan- 


216  STELLAR  EVOLUTION 

ism  that  moves  the  spectrum  across  the  bolometer  causes 
this  plate  to  travel  slowly  downward.  Thus  the  deflections 
of  the  needle  are  photographically  registered  upon  the  plate. 
With  the  aid  of  such  curves  the  total  atmospheric  absorption, 
measured  separately  for  each  region  of  the  spectrum,  is 
accurately  determined.  The  reduced  pyrheliometer  readings, 
corrected  in  this  way  for  absorption,  give  the  value  of  the 
solar  constant. 

With  such  highly  developed  instruments  the  systematic 
study  of  the  solar  radiation  was  pursued  in  Washington.  On 
the  best  days,  which  came  none  too  often,  the  refinement  of 
the  method  permitted  the  atmospheric  absorption  to  be  elimi- 
nated, even  at  this  station  so  near  the  level  of  the  sea.  It 
was  soon  found  that  the  values  of  the  "solar  constant"  were 
not  constant,  but  variable.  Indeed,  differences  as  great  as 
10  per  cent,  of  the  whole  were  encountered.  Was  it  safe  to 
conclude  that  the  solar  radiation  undergoes  variations  of  this 
considerable  amount? 

On  Mount  Wilson  the  escape  from  the  denser  air  of  the 
valley,  the  purity  of  the  upper  sky,  and  the  constant  succes- 
sion of  perfectly  clear  days,  permitted  the  question  to  be 
put  to  the  test.  Day  after  day  the  Sun  was  followed  through 
the  heavens,  from  a  time  soon  after  it  rose  above  the  eastern 
mountains  to  its  culmination  near  the  zenith.  Sometimes 
the  work  was  continued  through  the  afternoon,  but  the  morn- 
ing observations  proved  to  be  sufficient. 

As  soon  as  the  curves  had  been  measured  and  reduced, 
and  the  pyrheliometer  observations  plotted,  the  full  advan- 
tages of  the  mountain  station  appeared.  Not  only  was  the 
precision  of  the  work  much  greater  than  before:  even  more 
important  was  the  fact  that  daily  observations,  continued  for 
many  weeks,  brought  the  exact  nature  of  the  phenomenon  to 
light.  Through  the  latter  part  of  the  month  of  July,  1905, 
the  value  of  the  solar  constant  increased  slightly  from  day 


DOES  THE  SOLAR  HEAT  VARY?       217 

to  day,  until  it  reached  a  maximum.  It  then  declined  in  the 
same  gradual  manner.  From  these  results  Abbot  concluded 
that  the  solar  heat  had  temporarily  undergone  actual  change, 
not  to  be  ascribed  to  any  modification  of  our  own  atmosphere. 

Does  this  mean  a  greater  outpouring  of  the  solar  radia- 
tion, caused  by  an  actual  increase  in  the  surface  temperature 
of  the  Sun?  Or  had  the  absorption  of  the  solar  atmosphere 
decreased  for  a  time,  returning  later  to  its  normal  value? 
Much  study  will  be  required  to  answer  this  question,  though 
the  uncertainties  may  be  partially  cleared  up  when  the  1906 
observations  have  been  reduced.  Increased  solar  activity, 
represented  by  numerous  sun-spots  and  flocculi,  may  prob- 
ably be  taken  to  indicate  the  existence  of  more  numer- 
ous and  more  violent  convection  currents,  bringing  larger 
quantities  of  heat  from  the  Sun's  interior  to  the  surface. 
At  times  of  great  solar  activity,  therefore,  we  might  expect 
increased  radiation.  But  this  might  soon  be  checked  by 
the  diffusion  through  the  solar  atmosphere  of  materials 
thrown  upward  by  the  violent  eruptions,  which  characterize 
such  periods  of  activity.  Indeed,  the  increased  absorption, 
persisting  after  the  subsidence  of  unusual  activity,  might 
result  in  a  reduction  of  the  radiation  below  its  normal  value. 

Evidently  a  comparison  must  be  made  between  observa- 
tions of  various  kinds,  carried  on  simultaneously.  Spectro- 
heliograph  plates,  bearing  the  record  of  the  area  covered  by 
the  flocculi,  afford  an  index  to  the  solar  activity.  The 
absorption  of  the  solar  atmosphere  may  also  be  measured  by 
allowing  the  solar  image  to  drift  slowly  across  a  bolometer, 
and  photographing  the  galvanometer  deflections  upon  a  fall- 
ing plate.  During  the  summer  of  1906  both  of  these  classes 
of  work  were  carried  on  at  Mount  Wilson,  simultaneously 
with  Abbot's  measurements  of  the  solar  constant.  When  all 
the  results  are  discussed  together,  new  light  may  be  thrown 
on  the  subject. 


218  STELLAR  EVOLUTION 

But  the  work  is  barely  started,  and  must  be  continued  foi 
many  years  under  the  best  conditions.  Simultaneous  obser- 
vations at  several  widely  separated  mountain  stations  are 
greatly  to  be  desired,  to  make  certain  that  local  changes  in 
our  own  atmosphere  are  in  no  wise  concerned  in  the  apparent 
solar  changes.  Moreover,  the  work  should  go  on  without 
the  interruptions  caused  by  the  rainy  season.  If,  for  example, 
a  bolographic  outfit  were  established  at  the  Solar  Observatory 
at  Kodaikanal,  in  south  India,  at  an  elevation  of  7,000  feet, 
the  dry  season  there  would  correspond  with  the  rainy  season 
in  southern  California.  An  Australian  station  might  also 
accomplish  very  important  results.  It  is  to  be  hoped  that- 
adequate  provision  may  soon  be  made  to  carry  out  this  im- 
portant work. 

But,  it  may  be  asked,  must  not  such  fluctuations  of  the 
solar  radiation,  if  real,  be  the  cause  of  marked  changes  of 
terrestrial  temperature,  easily  detected  and  obvious  in  their 
effects?  Abbot  believes  that  the  thermometric  records  do 
actually  reflect  these  solar  variations,  but  Newcomb  holds  the 
contrary  view.  It  is  evident  that  complex  meteorological 
phenomena  may  be  involved,  and  that  their  disentanglement 
may  require  long-continued  research.  For  this  reason  the 
studies  of  the  solar  radiation  undertaken  by  the  Interna- 
tional Union  for  Co-operation  in  Solar  Research,  the  co- 
operation in  meteorological  work  set  on  foot  by  the  Solar 
Commission,  and  the  labors  of  such  an  institution  as  the 
observatory  recently  established  on  Mount  Weather,  Vir- 
ginia, by  the  United  States  Weather  Bureau,  should  prove 
of  value.  In  the  exhaustive  study  of  so  important  a  problem 
the  cordial  co-operation  of  many  investigators  is  essential  to 
success. 


CHAPTER  XXIII 

THE  CONSTRUCTION  OF  A  LARGE  REFLECTING 
TELESCOPE 

THE  grinding  and  polishing  of  a  60-inch  mirror  involve 
a  variety  of  operations,  described  in  detail  in  Ritchey's 
memoir  On  the  Modern  Reflecting  Telescope  and  the 
Making  and  Testing  of  Optical  Mirrors,1  the  most  authori- 
tative treatise  on  the  subject.  A  brief  account  of  these 
operations,  taken  in  large  part  from  the  above  source,  may 
be  of  interest  here. 

It  is  first  necessary  to  obtain  a  suitable  disk  of  glass. 
The  disk  (of  plate  glass)  made  by  the  French  Plate  Glass 
Works,  of  St.  Gobain,  France,  for  the  reflecting  telescope  of 
the  Solar  Observatory  is  60  inches  in  diameter,  8  inches 
thick,  and  weighs  a  ton.  It  must  be  remembered  that  the 
requirements  for  a  large  mirror  are  very  different  from  those 
for  a  lens  through  which  light  is  to  pass.  The  mirror  disk 
is  merely  a  support  for  the  thin  silver  film  on  its  front  sur- 
face, from  which  the  light  is  reflected  without  entering  the 
glass.  For  this  reason  the  great  perfection  of  a  lens  disk  is 
not  necessary.  Nevertheless,  the  glass  must  be  free  from 
striae  and  other  evidences  of  irregularity  of  structure.  It 
should  contain  no  large  bubbles,  though  a  few  small  ones,  if 
they  do  not  lie  on  the  surface,  are  not  objectionable.  The 
most  important  condition,  however,  is  freedom  from  strain 
caused  by  imperfect  annealing.  Evidences  of  strain  are 
detected  by  a  test  with  polarized  light.  Such  a  test,  how- 
ever, cannot  be  final,  as  an  incident  in  the  history  of  a  great 
telescope  objective  illustrates.  The  disk  had  been  carefully 

i  Published  by  the  Smithsonian  Institution- 

219 


220  STELLAR  EVOLUTION 

annealed  and  was  supposed  to  be  suitable  for  its  purpose. 
During  the  process  of  grinding  it  flew  to  pieces,  on  account 
of  internal  strain,  the  serious  nature  of  which  had  not  been 
recognized  in  the  test  with  polarized  light. 

It  may  not  be  obvious  why  the  disk  must  be  so  thick, 
when  its  sole  purpose  is  to  support  the  thin  film  of  silver 
on  its  accurately  figured  face.  Great  thickness,  however, 
is  absolutely  essential,  to  diminish  the  effects  of  bending 
due  to  the  weight  of  the  glass  and  to  temperature  changes. 
The  thickness  of  a  mirror  should  not  be  less  than  one- 
eighth  or  one-seventh  of  the  diameter.  Even  with  such 
thickness  a  special  support  system  is  necessary  to  prevent 
flexure. 

Glass  is  chosen  in  preference  to  other  materials  for  tele- 
scope mirrors  because  of  its  uniformity  of  structure,  com- 
parative ease  of  working,  and  capacity  for  a  high  polish. 
Its  lightness,  when  compared  with  such  substances  as  specu- 
lum metal  (formerly  employed  for  telescope  mirrors),  is 
an  important  advantage.  Furthermore,  a  surface  of  pure 
silver,  first  used  by  Foucault,  reflects  a  much  larger  propor- 
tion of  light  than  polished  speculum  metal. 

The  grinding-machine,  designed  and  constructed  by 
Ritchey  for  his  work  on  the  60-inch  mirror,  is  shown  in 
Plate  XCII.  The  glass  disk  rests  on  a  heavy  cast-iron  turn- 
table, carried  by  a  vertical  steel  shaft.  Between  the  lower 
surface  of  the  glass  (ground  flat)  and  the  turn-table  are  two 
thicknesses  of  Brussels  carpet,  which  form  an  admirable 
support  during  the  grinding  and  polishing  process.  The 
edge  of  the  glass  is  ground  true  by  means  of  a  rapidly 
rotating  iron  face-plate,  held  against  the  disk  while  the  turn- 
table is  slowly  rotated.  The  cutting  material  is  powdered 
carborundum,  carried  down  between  the  glass  and  the  face- 
plate by, a  slow  stream  of  water.  After  the  edge-grinding 
is  completed,  the  two  faces  of  the  glass  are  ground  plane 


A  LARGE  KEFLECTING  TELESCOPE  221 

and  parallel,  before  the  process  of  making  one  of  these  sur- 
faces concave  is  undertaken. 

The  grinding-tools  employed  for  this  work  are  circular 
plates  of  cast-iron,  strongly  ribbed  on  the  back,  and  divided 
into  a  series  of  small  squares  on  the  grinding  surface,  by 
two  sets  of  parallel  grooves,  planed  at  right  angles  to  one 
another.  The  tool  rests  on  the  surface  of  the  glass,  though 
in  Plate  XCIII  it  is  shown  suspended  from  the  lever  arm, 
employed  to  swing  the  heavy  tools  into  or  out  of  position. 
During  the  grinding  the  disk  is  slowly  rotated  arid  the  tool, 
also  kept  in  rotation,  is  moved  over  its  surface  in  a  series 
of  strokes  from  four  to  eight  inches  in  length,  by  means 
of  the  -arm  shown  above  the  disk  in  Plate  XCIII.  On  its 
right-hand  extremity  this  arm  terminates  in  a  steel  shaft, 
which  moves  back  and  forth  through  a  swiveled  bearing 
supported  on  an  adjustable  slide.  In  this  way  the  position 
of  the  grinding-tool  on  the  disk  can  be  changed  laterally, 
so  as  to  bring  the  stroke  across  the  center  of  the  glass  or 
near  the  edge.  If  it  is  found,  for  example,  that  the  center 
is  being  cut  away  too  rapidly,  the  tool  is  moved  near  the 
edge  and  the  grinding  continued  there  until  the  error  is 
corrected.  The  tool  is  not  kept  at  any  one  position  for  a 
great  length  of  time,  to  avoid  producing  low  zones  in  the 
glass. 

For  the  grinding  process,  various  grades  of  carborundum 
are  prepared  in  the  following  way:  The  powdered  carbo- 
rundum is  mixed  with  water  and  thoroughly  stirred.  After 
settling  for  two  minutes  the  coarse  particles  reach  the  bottom 
of  the  bucket  and  the  liquid,  containing  "two-minute"  car- 
borundum and  the  finer  grades,  is  siphoned  off  into  another 
bucket.  After  the  contents  of  the  second  bucket  have  been 
allowed  to  stand  four  minutes,  the  liquid  is  poured  off  and 
the  "two-minute"  carborundum  at  the  bottom  of  the  bucket  is 
set  aside  for  fine  grinding  purposes.  In  the  same  way,  carbo- 


222  STELLAR  EVOLUTION 

rundum  which  has  remained  in  suspension  for  periods  up  to 
one  hundred  and  twenty  minutes,  or  even  longer,  is  prepared, 
These  very  fine  grinding  materials  are  used  to  give  the 
smooth  and  almost  polished  surface  obtained  after  the  grind- 
ing with  coarser  carborundum  is  completed. 

A  perfectly  true  Brown  &  Sharpe  steel  straight-edge  is 
used  to  determine  whether  the  surface  of  the  glass  is  approx- 
imately plane.  When  it  is  found  to  be  sufficiently  so  for 
the  preliminary  work,  the  fine  grinding  is  commenced, 
beginning  with  two-minute  carborundum  and  continuing 
with  finer  grades.  In  this  work  the  iron  grinding-tool  is 
counter-poised  by  placing  weights  on  a  lever  arm  con- 
nected by  a  shaft  with  the  tool.  The  pressure  is  reduced 
from  one-third  pound  to  the  square  inch  for  the  five-  or  ten- 
minute  carborundum,  to  about  one-twelfth  pound  per  square 
inch  for  the  one-hundred-and-twenty-  and  two-hundred-and- 
forty-minute  carborundum.  Unless  this  precaution  is  taken 
there  is  great  danger  of  scratching  the  glass. 

After  being  fine  ground,  the  back  of  the  mirror  is 
polished  with  rouge  in  the  manner  described  later.  No 
great  pains  are  taken  with  this  surface,  although  it  is  made 
very  nearly  plane,  and  is  then  polished  so  as  to  permit  silver- 
ing (Plate  XCIV).  It  is  desirable  to  silver  the  back  of  the 
mirror,  as  well  as  the  front,  in  order  to  prevent  temperature 
changes  from  affecting  the  two  surfaces  in  unequal  degree. 

The  front  surface,  after  it  has  been  given  a  plane  figure, 
is  ready  to  be  made  concave.  For  this  purpose  a  convex 
iron  tool,  of  suitable  curvature,  is  employed.  In  the  case  of 
the  60-inch  mirror  the  radius  of  curvature  is  50  feet.  The 
curvature  of  the  tool,  and  also  of  the  glass,  is  tested  from 
time  to  time  by  a  spherometer.  This  consists  of  a  tripod, 
with  a  micrometer  screw  at  its  center,  which  permits  the 
deviation  of  the  surface  from  a  plane  to  be  accurately  deter- 
mined. After  the  desired  curvature  has  been  secured,  the 


A  LARGE  REFLECTING  TELESCOPE     223 

fine  grinding  is  carried  to  a  point  where  the  surface  is  very 
smooth  and  ready  for  polishing. 

The  polishing  and  figuring  are  done  by  means  of  a  tool 
built  up  of  narrow  strips  of  wood,  saturated  with  paraffine  to 
prevent  change  of  figure.  The  face  of  this  tool  is  covered 
with  squares  of  rosin,  of  a  certain  degree  of  hardness,  which 
can  be  determined  only  by  experience.  The  rosin  squares 
are  finally  coated  with  a  thin  layer  of  beeswax,  which  forms 
the  polishing  surface.  The  soft  wax  is  very  useful,  since 
small  hard  particles  that  may  happen  to  be  present  in  the 
polishing  material  are  likely  to  bed  themselves  in  it,  thus 
reducing  the  danger  of  scratches.  As  a  preliminary  to 
polishing,  the  tool  is  placed  in  contact  with  the  glass  disk 
and  pressed  against  it,  by  weights  placed  on  the  back,  so 
that  it  may  acquire  the  same  curvature  as  the  surface. 
After  pressing  for  some  hours,  until  the  waxed  squares 
appear  smooth  and  bright  in  all  parts,  the  polishing  may 
begin.  This  is  accomplished  by  moving  the  tool  over  the 
rotating  glass,  by  the  main  arm  of  the  machine,  as  in  the 
case  of  the  grinding  process.  The  polishing  material  is 
powdered  jewelers'  rouge,  used  commercially  for  polishing 
plate  glass.  The  fine  rouge  is  separated  from  impurities 
and  coarser  particles  by  a  washing  process  similar  to  that 
used  for  carborundum.  The  rouge,  mixed  with  distilled 
water,  is  applied  to  the  surface  of  the  glass  by  means  of  a 
wide  brush  of  cheese-cloth. 

The  greatest  precautions  must  be  taken  throughout  the 
polishing  process  to  avoid  scratches.  For  this  purpose  the 
room  in  which  the  work  is  done  is  fitted  up  in  such  a  way  as 
to  eliminate  danger  from  dust.  In  the  polishing-rooms  of 
the  Solar  Observatory  optical  shop  (Plate  XCV)  the  plas- 
tered walls  and  ceilings  are  heavily  varnished,  and  a  canvas 
screen  is  hung  above  the  glass,  to  protect  it  from  any  falling 
particles.  The  cement  floor  is  painted,  and  kept  wet  when  the 


224  STELLAR  EVOLUTION 

polishing  is  in  progress.  The  windows  are  double  and  care- 
fully sealed,  outer  air  being  admitted  to  the  room  through  a 
cheese-cloth  filter.  The  temperature  is  maintained  constant, 
within  two  or  three  degrees,  by  means  of  a  hot-water  furnace, 
controlled  by  a  thermostat.  The  motor,  driving-shaft,  and 
apparatus  for  varying  the  speed  of  the  griiidiiig-machine, 
are  carefully  inclosed,  only  the  slow-moving  belt  coming  out 
into  the  room.  No  one  is  permitted  to  enter  the  room  except 
the  optician,  who  wears  a  surgeon's  gown  and  cap.  By 
observing  such  precautions  the  work  may  be  continued  for 
months  without  producing  even  microscopic  scratches  in  the 
glass  surface. 

We  may  now  assume  that  the  glass  has  been  polished, 
after  receiving  an  approximately  spherical  surface.  It  then 
becomes  necessary  to  apply  a  more  accurate  test  than  the 
spherometer  permits.  For  this  purpose  the  glass  is  turned 
into  a  nearly  vertical  position,  where  it  is  supported  by  a  steel 
edge-band  (Plate  XCV).  An  artificial  star,  consisting  of 
a  hole  about  -g--^¥  of  an  inch  in  diameter  illuminated  by  an 
acetylene  lamp  or  other  brilliant  source  of  light,  is  placed  at 
the  center  of  curvature,  50  feet  from  the  glass  surface.  The 
light  from  the  articifial  star  then  falls  upon  the  disk  and  is 
reflected  back  so  as  to  form  an  image  close  beside  the  pin- 
hole.  If  the  surface  is  perfectly  spherical,  it  will  appear, 
when  examined  by  the  eye  placed  at  this  point,  to  be  bril- 
liantly and  uniformly  illuminated.  With  an  eye-piece,  the 
image  of  the  pin-hole  will  then  be  perfectly  sharp,  showing 
the  most  minute  details  or  irregularities  of  the  hole  itself. 

It  is  much  more  probable,  however,  that  the  surface  will 
have  many  zonal  errors.  To  detect  and  interpret  these,  the 
"knife-edge  test,"  due  to  Foucault,  is  employed.  If  all  the 
zones  come  to  a  focus  at  the  same  point,  and  a  knife  edge  is 
moved  across  this  point,  while  looking  at  the  glass,  the  light 
will  be  cut  off  instantly  from  all  parts  of  the  disk.  If,  how- 


A  LAEGE  KEFLECTING  TELESCOPE     225 

ever,  the  curvature  of  certain  zones  is  greater  or  less  than 
the  average  curvature,  these  zones  will  resemble  projecting 
or  receding  rings  on  an  otherwise  uniformly  bright  surface. 
The  effect  is  as  though  the  light  were  shining  from  one  side, 
producing  an  appearance  of  relief  by  lights  and  shadows. 
The  test  is  so  sensitive  that  an  error  of  -g-Wmnr  Par^  °^  an 
inch  can  be  detected.  If,  for  example,  the  finger  is  placed 
for  a  few  moments  on  the  glass,  the  heating  of  the  surface 
will  cause  a  swelling  easily  to  be  detected  by  the  knife-edge 
test. 

The  process  of  figuring  consists  in  removing  the  high 
and  low  zones  by  means  of  the  polishing  tool,  the  stroke 
and  position  of  which  must  be  modified  in  accordance  with 
the  results  of  the  knife-edge  test.  After  a  perfectly  spheri- 
cal form  has  been  obtained  in  this  way,  the  difficult  process 
of  changing  the  spherical  to  a  paraboloidal  surface  is  begun. 
As  is  well  known,  the  parallel  rays  from  a  star,  falling  on  a 
spherical  surface,  will  not  be  brought  to  a  focus  at  a  central 
point,  but  in  an  irregular  figure,  called  a  "caustic."  A 
paraboloid,  however,  brings  all  parallel  rays  to  a  single 
focus,  and  produces  a  perfect  stellar  image.  In  the  case  of 
the  60-inch  mirror,  which  has  a  focal  length  of  25  feet,  the 
paraboloid  is  deeper  than  the  sphere  at  the  center  of  the 
disk  by  a  quantity  less  than  T-f^-Q  of  an  inch.  Months  of 
figuring  are  required,  however,  to  produce  this  small  differ- 
ence, because  of  the  necessity  of  giving  each  zone  of  the 
paraboloid  precisely  the  right  curvature.  In  testing  the 
surface  from  the  center  of  curvature,  the  measured  radius  of 
each  narrow  zone  of  the  mirror  (the  other  parts  being 
covered  by  a  cardboard  screen)  must  correspond  with  the 
calculated  radius.  The  extreme  difficulty  of  accomplishing 
this  may  be  appreciated  when  it  is  remembered  that  the 
deviation  of  any  zone  from  the  surface  of  a  perfect  para- 
boloid must  not  be  greater  than  TTFFTFTnnr  °^  an  incn?  which 


226  STELLAR  EVOLUTION 

would   correspond   to  a  change  of   T^¥  of    an  inch  in  the 
radius  of  curvature. 

When  parallel  light  is  available,  the  difficulties  of  secur- 
ing a  perfectly  satisfactory  test  of  a  paraboloidal  mirror  are 
greatly  reduced.  In  this  case  the  mirror,  when  seen  from 
its  focal  plane  (25  feet  from  the  glass,  or  one-half  the 
radius  of  curvature)  appears  like  a  uniformly  illuminated 
plane  surface  when  a  perfectly  paraboloidal  form  has  been 
obtained.  This  method  of  testing  with  parallel  light  has 
been  developed  by  Ritchey,  and  was  used  by  him  to  secure 
the  last  degree  of  perfection  in  the  figure  of  the  60-inch 
mirror. 

As  already  explained,  the  problem  of  mounting  a  large 
mirror  is  quite  as  serious  as  that  of  figuring  it.  It  is  neces- 
sary, in  the  first  place,  to  support  the  mirror  in  such  a  way 
that  it  will  retain  its  form,  without  bending,  in  any  position 
of  the  telescope.  Furthermore,  it  must  be  held  so  that  it 
will  not  slip  laterally,  since  the  slightest  change  in  the  posi- 
tion of  the  mirror  with  respect  to  the  tube  will  cause  a  dis- 
placement of  the  star  images  on  the  photographic  plate. 
The  mirror,  thus  supported,  must  be  carried  at  the  lower 
end  of  a  tube,  of  skeleton  construction,  open  at  the  top,  and 
so  mounted  that  it  can  be  pointed  toward  any  part  of  the 
heavens  and  made  to  follow  the  apparent  motion  of  the 
stars  by  rotation  about  an  axis  parallel  to  the  axis  of  the 
Earth.  Strength  and  stability  of  the  mounting,  freedom 
from  flexure,  perfection  of  optical  and  mechanical  construc- 
tion and  adjustment,  and  the  greatest  precision  of  driving  — 
all  these  conditions  must  be  met  before  a  large  reflector  can 
be  expected  to  give  satisfactory  results,  in  the  more  exacting 
departments  of  photographic  work. 

The  difficulties  thus  presented  have  been  most  successfully 
solved  by  Ritchey,  whose  design  for  the  mounting  of  the 
60-inch  mirror  is  shown  in  Plate  XCVI.  The  telescope  tube 


A  LARGE  REFLECTING  TELESCOPE  227 

is  hung  between  the  arms  of  a  massive  cast-iron  fork,  which 
is  bolted  to  the  upper  end  of  the  polar  axis.  This  axis,  a 
hollow  forging  of  nickel  steel,  is  inclined  at  an  angle  corre- 
sponding to  the  latitude  of  Mount  Wilson  (34°  13')  and 
thus  rendered  parallel  to  the  axis  of  the  Earth.  Leveling 
screws,  by  which  the  base  of  the  mounting  is  supported  on 
its  pier,  permit  this  adjustment  to  be  made  with  great  pre- 
cision. In  order  to  relieve  the  great  friction  of  this  axis  on 
the  upper  and  lower  bearings  in  which  it  lies,  a  hollow  steel 
float,  10  feet  in  diameter,  is  bolted  to  its  upper  end,  just 
below  the  fork.  This  float  dips  into  a  tank  filled  with  mer- 
cury. Thus  the  entire  instrument  is  floated  by  the  mercury, 
and  in  this  way  the  friction  on  the  bearings  is  reduced  to  a 
minimum. 

The  60-inch  mirror  rests  at  the  lower  end  of  the  tube,  on 
a  support  system  consisting  of  a  large  number  of  weighted 
levers,  which  press  against  the  back  of  the  glass  and  dis- 
tribute the  load.  A  similar  series  of  weighted  levers  around 
the  circumference  of  the  mirror  provide  the  edge  support. 
The  path  of  the  rays  from  the  star  may  be  as  shown  in 
Plate  XCVII,  Figs.  1,  2,  3,  or  4.  In  the  first  arrangement 
(the  Newtonian  telescope),  the  parallel  rays,  after  striking 
the  mirror,  are  reflected  back  and  would  come  to  a  focus  at  a 
point  just  beyond  the  end  of  the  tube.  They  are  intercepted, 
however,  by  a  plane  mirror  of  silvered  glass,  which  turns 
them  at  right  angles  and  forms  the  image  on  the  photo- 
graphic plate,  which  is  mounted  on  the  side  of  the  tube  near 
the  upper  end.  In  this  case  the  focal  length  of  the  instru- 
ment is  25  feet,  and  the  image  is  formed  without  secondary 
magnification. 

If,  however,  it  is  desired  to  secure,  for  certain  classes  of 
work,  the  advantages  of  a  greater  focal  length,  a  different 
arrangement  is  adopted.  The  upper  section  of  the  tube, 
bearing  the  plane  mirror,  is  removed,  and  a  shorter  section 


228  STELLAR  EVOLUTION 

substituted  for  it.  This  carries  a  hyperboloidal  mirror, 
which  returns  the  rays  toward  the  center  of  the  large  mirror 
and  causes  them  to  converge  less  rapidly.  They  then  meet 
a  small  plane  mirror,  supported  at  the  middle  of  the  tube 
near  its  lower  end,  which  sends  them  to  one  of  the  following 
instruments,  mounted  in  the  focal  plane:  (1)  a  double-slide 
plate-holder,  carrying  a  sensitive  plate,  for  the  photography 
of  the  Moon,  planets,  bright  nebulae,  etc.,  with  an  equivalent 
focal  length  of  100  feet  (Fig.  3) ;  (2)  a  spectrograph  mounted 
in  place  of  this  photographic  plate,  in  which  case  a  convex 
mirror  of  different  curvature  is  employed,  and  the  equivalent 
focal  length  is  80  feet  (Fig.  4) ;  or  finally  (3)  a  third  convex 
mirror  may  be  used  and  the  plane  mirror  inclined  so  as  to 
form  the  star  image  (after  sending  the  light  down  through 
the  hollow  polar  axis)  on  the  slit  of  a  powerful  spectrograph, 
of  13  feet  focal  length,  mounted  on  a  pier  in  a  constant- 
temperature  chamber  (Fig.  2).  In  this  case  the  equivalent 
focal  length  is  150  feet. 

The  telescope  is  moved  in  right  ascension  or  declination 
by  electric  motors,  controlled  from  the  floor  of  the  observing- 
room.  The  driving-clock  moves  the  telescope  in  right  ascen- 
sion by  means  of  a  worm-gear,  10  feet  in  diameter,  carried 
by  the  polar  axis.  The  cutting  of  the  teeth  of  this  worm- 
gear  is  a  mechanical  operation  requiring  the  highest  precision 
of  workmanship.  Each  tooth  was  spaced  off  by  means  of  a 
finely  divided  circle  attached  to  the  polar  axis,  and  read  with 
a  microscope.  The  rotating  cutter  was  driven  by  an  electric 
motor.  After  all  the  teeth  had  been  cut,  the  worm  and  worm- 
gear  were  ground  together  for  many  hours,  until  all  slight 
residual  errors  had  been  eliminated.  The  operation  was 
completed  with  jewelers'  rouge,  which  leaves  a  smooth  and 
highly  polished  surface. 

All  of  the  heavy  parts  of  this  mounting  were  made,  after 
Ritchey's  designs,  by  the  Union  Iron  Works  Company,  of 


A  LARGE  REFLECTING  TELESCOPE     229 

San  Francisco.  They  were  then  shipped  to  Pasadena,  where 
the  mounting  has  been  erected  in  the  Solar  Observatory  shop 
(Plate  XCVIII).  Here  the  worm-gear  was  cut,  and  all  of 
the  smaller  parts,  including  the  driving-clock,  setting-circles, 
slow  motions,  motors,  etc.,  are  being  fitted  and  adjusted. 
All  of  these  parts  were  made  in  the  Observatory  instrument 
shop,  which  is  equipped  with  the  best  machinery  obtainable 
for  work  of  this  kind  (Plate  XOIX). 

As  soon  as  this  mounting  has  been  completed,  the  60-inch 
mirror  will  be  put  in  place  and  the  telescope  thoroughly 
tested,  by  actual  photography  of  the  heavens.  It  will  then 
be  necessary  to  transport  the  instrument  to  Mount  Wilson — 
an  operation  of  considerable  difficulty,  as  several  of  the  cast- 
ings are  very  large,  and  weigh  about  five  tons  each. 

The  building  for  the  60-inch  reflector  is  of  steel  con- 
struction throughout  (Plate  C).  The  thin  inner  walls  will 
be  shielded  from  the  Sun  by  outer  walls,  and  air  will  be  per- 
mitted to  circulate  in  the  space  between  the  two.  The  dome, 
60  feet  in  diameter,  will  be  rotated  by  an  electric  motor, 
either  rapidly,  when  passing  from  one  part  of  the  heavens 
to  another,  or  at  a  slow,  uniform  rate,  of  such  a  speed  as 
to  keep  the  opening  (15  feet  wide)  constantly  opposite  the 
end  of  the  telescope  tube,  when  it  is  following  a  star.  The 
observer,  when  photographing  in  the  principal  focus,  will 
stand  on  a  platform  suspended  from  the  dome  and  rotating 
with  it.  The  double-slide  plate-carrier,  with  which  stars 
and  nebulae  will  be  photographed,  is  similar  to  that  used 
with  the  Yerkes  telescope  (Plate  XVII). 


CHAPTER  XXIV 
SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS 

IN  looking  toward  the  future  and  endeavoring  to  imagine 
what  appliances  will  be  employed  by  the  astronomer  of  the 
next  generation,  the  line  of  least  resistance  is  to  consider  the 
possibilities  of  improving  existing  telescopes  and  the  auxil- 
iary apparatus  employed  with  them;  for  the  prevision  of 
more  radical  departures  is  beyond  our  province.  It  is  safe 
to  predict  that  the  equatorial  refractor,  of  which  the  Lick 
and  Yerkes  telescopes  are  types,  will  hold  an  important 
place  in  observatories  for  many  years  to  come.  The  ease 
with  which  such  instruments  can  be  pointed  toward  any  part 
of  the  heavens;  the  absence  of  reflecting  surfaces;  the  per- 
manence of  object-glasses,  as  contrasted  with  the  necessity 
of  silvering  mirrors  from  time  to  time ;  the  convenient  posi- 
tion of  the  observer  at  the  lower  end  of  the  tube,  rather  than 
at  the  upper  end  of  a  Newtonian  reflector:  these  and  other 
considerations  point  to  the  long-continued  use  of  the  standard 
refractor.  In  its  most  perfect  form  this  instrument  is  still 
capable  of  some  improvements,  the  most  important  of  which 
will  be  the  introduction  of  truly  achromatic  object-glasses, 
capable  of  uniting  the  rays  of  all  colors  at  the  same  focus. 

It  seems  probable  that  the  uses  of  the  equatorial  refractor 
will  be  confined  more  and  more  to  visual  observations,  and 
to  certain  departments  of  photography,  especially  those 
involving  great  precision  of  measurement  or  the  inclusion 
of  large  fields  on  a  single  plate.  For  the  latter  work  the 
refracting  telescope,  particularly  in  the  portrait-lens  form, 
possesses  great  advantages,  on  account  of  the  very  limited 
field  of  the  reflector.  It  does  not  at  present  appear  desir- 

230 


SOME  POSSIBILITIES  or  NEW  INSTRUMENTS    231 

able  to  increase  the  aperture  of  refractors  beyond  the  limit 
of  40  inches  reached  in  the  Yerkes  telescope.  The  resolving 
power  of  such  an  aperture,  when  the  atmospheric  condi- 
tions are  good  enough  to  permit  its  realization,  is  suffi- 
ciently great  for  the  most  exacting  demands  of  visual  work. 
Increased  light-gathering  power,  which  is  much  to  be  desired 
for  the  investigation  of  faint  objects,  will  be  most  easily  and 
effectively  obtained  through  the  use  of  large  reflecting  tele- 
scopes. Increased  focal  length,  on  the  other  hand,  which  is 
needed  to  give  larger  solar  images,  can  best  be  secured 
through  the  use  of  some  form  of  fixed  telescope.  We  may 
now  consider  what  types  of  telescopes  are  likely  to  prove 
most  serviceable  in  photographic  and  spectroscopic  studies 
of  the  Sun,  stars,  and  nebulae. 

Many  important  investigations  require  the  use  of  a  tele- 
scope giving  a  sharply  defined  solar  image,  of  large  diameter, 
at  a  fixed  position  within  a  laboratory.  The  focal  length  of 
such  a  telescope  must  not  change  rapidly  when  the  instru- 
ment is  exposed  to  the  Sun.  The  image  must  not  rotate, 
and  the  laboratory  conditions  must  permit  the  successful  use 
of  the  largest  and  most  powerful  spectrographs  and  spec- 
troheliographs.  The  Snow  telescope  meets  most  of  these 
requirements  in  a  very  satisfactory  manner.  The  one  diffi- 
culty with  this  instrument  is  the  distortion  of  the  image  and 
the  change  of  focus  when  the  mirrors  are  exposed  for  some 
time  to  the  Sun.  When  the  precautions  described  in  chap, 
xv  are  taken,  these  obstacles  are  easily  overcome  in  cur- 
rent work  with  the  5 -foot  spectroheliograph  and  the  Littrow 
spectrograph.  But  with  long  exposures,  such  as  are  required 
with  a  spectroheliograph  of  very  high  dispersion,  the  change 
of  focus  during  the  exposure  would  be  a  serious  obstacle.  It 
is  probable  that  by  substituting  very  thick  mirrors  for  those 
now  used  in  the  Snow  telescope,  and  by  reflecting  sunlight 
upon  their  rear  surfaces,  which  should  be  silvered  like  the 


232  STELLAE  EVOLUTION 

front  surfaces,  the  tendency  to  distortion  could  be  overcome. 
For  a  very  thick  mirror  would  resist  the  bending  which 
results  from  the  expansion  of  the  front  surface;  and  even  if 
the  figure  were  changed,  the  compensating  effect  produced 
by  heating  the  rear  surface  should  restore  it.  But  the  Snow 
telescope  is  fully  occupied  with  its  present  work,  for  which 
it  is  well  adapted.  Accordingly,  a  new  type  of  fixed  tele- 
scope has  been  devised  for  the  purpose  of  supplementing  the 
Snow  telescope,  particularly  in  photographic  work  involving 
long  exposures. 

In  the  new  instrument  the  coelostat,  provided  with  mir- 
rors a  foot  thick,  will  be  mounted  at  the  summit  of  a  steel 
tower  65  feet  in  height  (Fig.  7).  From  the  second  mirror 
the  sunlight  will  be  sent  vertically  downward  to  a  12-inch 
object-glass,  mounted  a  short  distance  below  it.  This  object- 
glass,  of  60  feet  focal  length,  will  form  an  image  of  the  Sun 
near  the  ground  level.  The  new  instrument  will  thus  consist 
essentially  of  a  fixed  refracting  telescope,  pointing  directly 
to  the  zenith  and  receiving  light  from  a  coelostat  and  second 
mirror. 

The  spectroscopic  laboratory  at  the  base  of  the  tower  will 
be  excavated  in  the  earth,  to  insure  constancy  of  temperature 
and  great  stability  of  the  instruments  it  will  contain.  Of 
these,  the  one  shown  on  the  left  in  Fig.  71  is  a  Littrow 
spectrograph,  similar  to  the  one  employed  with  the  Snow 
telescope,  but  of  much  greater  power.  This  instrument  will 
have  a  focal  length  of  30  feet,  and  be  provided  with  a  large 
plane  grating.  On  the  right  is  shown  a  spectroheliograph  of 
30  feet  focal  length,  designed  for  extending  the  monochromatic 
photography  of  the  Sun  to  many  of  the  finer  lines  of  the 
spectrum  (p.  236).  The  atmospheric  calm  that  prevails  on 

iThis  is  only  a  general  diagram,  omitting  all  details,  such  as  the  steel  house,  at 
the  base  of  the  tower,  which  covers  the  upper  ends  of  the  spectroscope  and  spectro- 
heliograph ;  the  small  electric  elevator,  to  convey  the  observer  from  the  bottom  of 
the  underground  laboratory  to  the  summit  of  the  tower,  etc. 


SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS    233 

Mount  Wilson  during  the  best  observing  season  may  permit 
the  inner  tower  to  be  used  merely  as  a  skeleton,  if  firmly 
stayed  in  position  by  strong  steel  guy-ropes.  If,  on  experi- 
ment, it  is  found  that  the  wind  produces  too  much  vibration 
of  the  structure,  an  outer  tower,  covered  with  canvas  louvers,1 
will  be  erected  to  shield  the  inner  one,  as  indicated  in  Fig.  7. 

It  remains  to  be  seen  whether  this  type  of  telescope  will 
meet  the  rigorous  conditions  demanded  in  the  case  of  a  fixed 
instrument  for  solar  research.  The  vertical  beam  of  light 
should  be  less  affected  by  unequal  temperature  conditions 
than  a  horizontal  beam,  and  the  considerable  height  of  the 
coelostat  and  object-glass  above  the  ground  may  also  prove 
advantageous.  Should  it  prove  successful,  a  similar  instru- 
ment of  larger  aperture,  and  of  about  150  feet  focal  length, 
may  ultimately  be  constructed,  on  account  of  the  importance 
of  providing  a  very  large  image  of  the  Sun  for  certain  classes 
of  spectroscopic  and  spectroheliographic  work.  It  is  prob- 
able enough  that  some  other  type  of  fixed  telescope  would  be 
better  than  this,  but  the  results  of  our  experience  up  to  the 
present  time  give  reason  for  the  belief  that  the  present  design 
will  prove  satisfactory. 

Since  the  principal  difficulty  to  be  overcome  in  the  con- 
struction of  a  fixed  telescope  for  solar  work  is  the  distortion 
of  the  mirrors  by  the  Sun's  heat,  it  is  to  be  hoped  that  homo- 
geneous disks  of  fused  quartz  can  ultimately  be  employed 
for  mirrors,  in  place  of  glass.  The  coefficient  of  expansion 
of  fused  quartz  is  only  about  one-tenth  that  of  glass,  and 
hence  it  is  but  slightly  subject  to  change  of  figure  by  heat. 
Many  small  quartz  disks  have  been  made  in  an  electric  fur- 
nace at  the  Solar  Observatory,  but  the  presence  of  numerous 
bubbles,  which  cannot  be  removed  from  the  very  viscous  fluid 
by  stirring,  have  proved  an  insuperable  obstacle  to  the  use 

1  Or  perhaps  with  flue  wire  netting,  which  should  break  the  wind,  and  yet  not 
iieat  sufficiently  in  sunlight  to  produce  convection  currents. 


234 


STELLAR  EVOLUTION 


FIG.  7 
Vertical  Coelostat  Telescope 


SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS    235 

of  these  disks  for  optical  purposes.  Day  has  met  with  better 
success  in  the  geophysical  laboratory  of  the  Carnegie  Insti- 
tution, where  an  electric  furnace  of  special  type  permitted 
quartz  to  be  fused  under  pressure.  His  results  are  suffi- 
ciently promising  to  lead  to  the  hope  that,  if  a  large  furnace, 
of  suitable  design,  were  constructed,  disks  of  15  to  20  inches 
in  diameter  might  be  made.  In  view  of  the  expense  of  such 
a  furnace,  it  has  seemed  best  to  defer  further  experiments 
in  this  direction  until  very  thick  glass  mirrors  can  be 
thoroughly  tested.1 

Another  important  need  of  the  future  is  a  machine  capable 
of  ruling  gratings  of  much  larger  dimensions  than  those  of 
Rowland.  The  best  Rowland  gratings,  which  have  rendered 
possible  the  great  advances  of  the  last  quarter-century  in 
spectroscopy,  have  a  ruled  surface  about  5J  inches  long. 
The  resolving  power  of  a  grating  depends  upon  the  total 
number  of  lines  it  contains,  but  there  are  many  reasons 
why  it  is  not  desirable  that  the  number  should  exceed  20,000 
per  inch.  If  a  good  15-  or  20-inch  grating  could  be  ruled, 
with  lines  from  10  to  15  inches  in  length,  a  great  advance 
in  solar  spectroscopy  would  be  rendered  possible.  Such  a 
grating,  if  one  of  the  spectra  were  very  brilliant,  would  be 
exactly  what  is  required  for  a  spectroheliograph  capable 
of  photographing  the  Sun  through  narrow  dark  lines. 
If  used  in  a  spectrograph  of  from  40  to  50  feet  focal 
length,  it  would  furnish  a  photographic  map  of  the  solar 
spectrum  much  superior  to  Rowland's,  and  be  of  the  greatest 
service  in  the  photography  of  sun-spot  spectra,  the  study  of 
the  solar  rotation,  and  many  other  investigations.  For  this 
purpose  the  spectra  of  one  of  the  higher  orders  (from  second 
to  fourth)  should  be  bright,  and  the  precision  of  ruling 

1  Since  the  above  was  written  the  "tower  telescope"  has  been  constructed  and 
t  sted.  The  thick  mirrors  are  so  little  affected  by  sunlight  that  the  focus  will 
remain  constant  during  the  long  exposures  required  with  the  30-foot  spectrohelio- 
graph (Plates  CI  and  CII). 


236  STELLAR  EVOLUTION 

should,  of  course,  be  so  high  as  to  permit  the  theoretical 
resolving  power  to  be  attained.  In  view  of  Michelson's 
recent  work  in  ruling  8-iiich  and  10-inch  gratings,  the 
realization  of  his  plan  for  the  construction  of  a  machine 
capable  of  making  gratings  of  much  larger  size  is  more 
earnestly  to  be  desired  than  that  of  any  other  project  for 
the  development  of  spectroscopy. 

Still  another  important  need  of  the  spectroscopist  is 
homogeneous  glass,  in  large  masses,  for  prisms.  At  the 
present  time  it  is  almost  impossible  to  obtain  prisms  of 
large  size  that  will  give  good  definition.  The  repeated 
failures  of  the  best  makers  of  optical  glass  indicate  that  the 
problem  is  not  an  easy  one,  though  it  can  probably  be 
solved.  A  careful  study  of  this  question,  made  with  special 
reference  to  the  possibility  of  improving  the  present  methods 
of  annealing,  should  yield  valuable  results.  Large  prisms 
are  urgently  required  for  use  in  stellar  spectrographs  of  large 
aperture  and  high  dispersion,  such  as  the  one  which  is  to  be 
mounted  in  a  constant-temperature  chamber  in  conjunction 
with  the  60-inch  reflector.  However,  if  sufficiently  large 
and  perfect  gratings  can  be  obtained,  which  concentrate 
nearly  all  of  the  light  in  a  single  spectrum,  they  may  be 
better  for  this  purpose  than  prisms. 

In  the  further  development  of  solar  research,  no  instru- 
ment seems  to  offer  more  possibilities  than  the  spectrohelio- 
graph.  Recent  experiments  with  a  temporary  spectrohelio- 
graph  of  30  feet  focal  length,  used  in  conjunction  with  the 
Snow  telescope,  have  demonstrated  the  feasibility  of  photo- 
graphing sun-spots  with  the  lines  that  are  strengthened  or 
weakened  in  their  spectra.  The  resulting  pictures  show  the 
distribution  of  the  corresponding  vapors  in  and  around  the 
spots,  and  should  be  capable  of  throwing  much  new  light 
on  solar  phenomena  when  taken  daily  and  systematically 
studied.  It  is  expected  that  the  30-foot  spectroheliograph 


SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS    237 

of  the  "tower  telescope"  will  be  employed  in  this  way,  but 
even  the  great  dispersion  of  this  instrument  will  be  inade- 
quate for  work  with  the  finest  lines.  It  is  evident  that  if 
the  photograph  is  to  represent  the  distribution  of  the  gas 
or  vapor  corresponding  to  the  line  employed,  the  line  must 
be  as  wide  as  the  second  slit,  in  order  that  light  from  the 
adjoining  continuous  spectrum  may  not  obliterate  or  confuse 
the  image  produced  by  it.  When  it  is  remembered  that  the 
solar  spectrum  contains  more  than  20,000  lines,  and  that 
any  one  of  these  may  be  capable  of  furnishing  a  photograph 
comparable  in  interest  with  the  results  already  obtained  with 
hydrogen  and  calcium  lines,  it  will  be  appreciated  that  no 
effort  should  be  spared  to  increase  the  dispersion  and  optical 
perfection  of  the  spectroheliograph.  The  further  applica- 
tions of  this  instrument  to  the  study  of  the  level  of  the 
fiocculi;  the  absorption  of  the  solar  atmosphere;  the  growth 
of  the  flocculi  and  prominences,  which  can  be  shown,  as  if 
in  accelerated  progress,  by  the  aid  of  a  series  of  pictures 
taken  in  rapid  succession  and  projected  on  a  screen  with  a 
kinematograph ;  the  use  of  stereoscopic  methods  in  spectro- 
heliographic  work:  these,  and  many  other  investigations, 
leave  no  doubt  that  this  field  of  solar  research  is  but  barely 
opened,  and  still  contains  many  untried  possibilities. 

Passing  over  other  considerations  that  tend  to  confirm 
one's  optimistic  belief  in  the  future  of  solar  research,  we  may 
now  inquire  as  to  the  type  of  telescope  that  appears  most 
promising  for  photographic  and  spectrographic  studies  of 
stars  and  nebulae.  In  much  of  this  work  it  is  not  essential, 
as  in  the  case  of  the  Sun,  that  the  image  should  be  fixed  in  a 
laboratory.  For  this  reason,  an  equatorially  mounted  reflect- 
ing telescope  seems  to  meet  the  requirements  admirably. 
Even  when  a  fixed  image  is  required,  it  is  possible,  as  illus- 
trated in  Fig.  2,  Plate  XCVII,  to  send  the  light  from  objects 
lying  within  a  certain  zone  of  the  heavens  into  a  constant- 


238  STELLAK  EVOLUTION 

temperature  laboratory,  for  analysis  by  the  most  powerful 
spectrographs.  As  already  explained,  such  a  telescope  is 
also  adapted  for  many  other  classes  of  work,  either  in  the 
principal  focus  of  the  great  mirror  or  with  an  enlarged 
image  given  by  a  convex  mirror,  after  the  manner  of  Casse- 
grain. 

As  an  object-glass  increases  in  size,  the  absorption,  due 
to  its  increased  thickness,  rapidly  diminishes  the  percentage 
of  light  it  transmits.  The  loss  is  especially  serious  for  the 
blue  and  violet  rays,  since  these  are  absorbed  more  completely 
than  the  red  and  yellow.  In  the  case  of  a  mirror,  the  light 
passes  through  no  glass,  but  falls  on  a  surface  of  pure  silver, 
from  which  it  is  reflected  to  the  focal  plane.  Thus  every 
square  inch  added  to  the  area  of  a  telescope  mirror  means 
a  proportional  increase  in  the  light-gathering  power.  It 
is  evident  that  if  the  mechanical  and  optical  difficulties 
can  be  overcome,  reflecting  telescopes  much  more  power- 
ful than  any  now  in  existence  can  advantageously  be  con- 
structed. 

With  this  object  in  view,  Mr.  John  D.  Hooker  has  pre- 
sented to  the  Carnegie  Institution  a  sum  sufficient  to  pur- 
chase for  the  Solar  Observatory  a  glass  disk  100  inches  in 
diameter  and  13  inches  thick,  and  to  meet  other  expenses 
incident  to  the  construction  of  a  100-inch  mirror  for  a  reflect- 
ing telescope  of  50  feet  focal  length.  The  construction  of  a 
telescope  so  far  surpassing  all  previous  instruments  in  size 
must,  of  course,  be  partly  in  the  nature  of  an  experiment. 
The  immense  block  of  glass  will  weigh  4J  tons,  four  and  one- 
half  times  as  much  as  the  disk  of  the  60-inch  mirror.  The 
difficulty  of  providing  a  mounting  capable  of  carrying  it  with 
the  necessary  precision  is  not  slight.  The  glass  is  certain  to 
be  more  or  less  distorted  by  temperature  changes,  which 
would  ruin  its  performance  if  not  obviated.  The  atmospheric 
conditions,  even  on  Mount  Wilson,  may  not  be  sufficiently 


SOME  POSSIBILITIES  OF  NEW  INSTRUMENTS    239 

good  to  permit  so  great  an  aperture  to  be  used  to  full  advantage. 
Of  these  and  other  obstacles  Mr.  Hooker  is  fully  informed, 
and  he  does  not  underestimate  their  importance.  But  he 
perceives  and  appreciates,  with  the  understanding  of  one  who 
has  himself  invented  and  developed  mechanical  appliances, 
that  experiment  is  necessary  to  progress.  He  therefore  does 
not  hesitate  to  provide  the  means  for  undertaking  an  optical 
experiment  on  a  large  scale.  Let  us  consider  its  probable 
outcome. 

In  the  first  place,  the  question  arises  whether  a  sufficiently 
homogeneous  glass  disk  of  the  required  dimensions  can  be 
obtained.  Our  long  experience  with  the  Plate  Glass  Com- 
pany of  St.  Gobain  (France)  leads  us  to  believe  that  110 
insuperable  difficulty  will  be  encountered.  This  old  and  reli- 
able company  has  cast  for  us  scores  of  disks,  from  which 
Ritchey  has  made  many  plane  and  concave  mirrors,  from  the 
smallest  sizes  up  to  60  inches.  In  all  of  these  cases  the 
quality  of  the  disks  has  left  nothing  to  be  desired.  The 
60-inch,  8  inches  thick,  and  weighing  a  ton,  is  fully  equal  to 
the  smaller  ones.  We  are  therefore  inclined  to  believe,  since 
the  St.  Gobain  Company  expresses  its  deliberate  opinion  that 
a  satisfactory  disk,  100  inches  in  diameter  and  13  inches 
thick,  can  be  produced,  that  they  will  be  able  to  carry  out 
the  order  we  have  given  them. 

As  for  the  work  of  grinding  and  figuring,  no  one  who 
has  watched  the  progress  of  the  60-inch  mirror  would  be 
likely  to  doubt  Ritchey's  ability  to  accomplish  this  difficult 
task.  The  method  of  parabolizing  which  he  has  perfected 
will  apply  as  well  to  a  100-inch  mirror  as  to  the  60-inch.  It 
eliminates  the  necessity  of  handwork,  except  for  a  few  finish- 
ing touches,  and  has  yielded  an  essentially  perfect  parabo- 
loidal  figure  in  the  case  of  the  60-inch  mirror.  I  am  con- 
fident that  he  will  find  no  difficulty  in  bringing  the  100-inch 
mirror  to  this  highest  order  of  perfection. 


240  STELLAR  EVOLUTION 

The  mounting  should  offer  no  great  obstacles,  especially 
as  it  will  not  be  built  until  the  mounting  of  the  60-inch 
has  been  thoroughly  tested  on  Mount  Wilson.  In  these 
days  of  large  and  perfect  machinery,  the  mechanical  diffi- 
culties are  much  less  formidable  than  they  would  have 
appeared  twenty  years  ago.  On  this  score,  therefore,  we 
see  no  cause  for  fear. 

The  prevention  of  change  of  figure  due  to  changing  tem- 
perature should  not  prove  a  very  serious  problem.  During 
the  fine  nights  of  the  best  observing  season  on  Mount  Wilson 
the  temperature  remains  almost  perfectly  constant  after  9  P.  M. 
It  will  therefore  only  be  necessary  to  maintain  the  mirror 
(or  possibly  the  entire  telescope)  at  approximately  this  tem- 
perature throughout  the  df  y ,  by  means  of  suitable  refrigerat- 
ing machinery.  In  the  long  periods  of  cloudless  weather  the 
change  of  temperature  from  night  to  night  is  extremely 
small,  so  that  little  difficulty  should  be  encountered  on  this 
score.  If  the  slowly  falling  temperature  during  the  early 
evening  should  prove  to  give  trouble,  the  observational  work 
may  be  deferred  until  after  nine  o'clock.  The  dome  and 
building,  like  those  for  the  60-inch  reflector,  will  be  so  con- 
structed that  no  air  can  enter  during  the  day ;  they  will  also 
be  shielded  from  the  heat  of  the  Sun.  The  problem  is,  of 
course,  altogether  different  from  that  encountered  in  the 
case  of  the  Snow  telescope,  where  the  mirrors  are  required 
to  give  good  images  in  spite  of  their  exposure  to  direct 
sunlight. 

Assuming  that  these  various  difficulties  can  be  success- 
fully overcome,  it  still  remains  a  question  whether  the  atmos- 
pheric conditions  on  Mount  Wilson  will  be  sufficiently  good 
to  permit  the  telescope  to  give  satisfactory  images.  This 
cannot  be  definitely  determined  until  after  the  60-inch  reflector 
has  been  used  for  some  time.  Even  if  it  should  prove,  how- 
ever, that  only  a  very  few  nights  in  the  course  of  a  year  can 


SOME  POSSIBILITIES  OF  NEW  INSTEUMENTS    241 

be  utilized  to  the  fullest  advantage,  the  construction  of  such 
a  telescope  would  nevertheless  be  desirable.  For  under  the 
average  summer  conditions,  which  are  much  finer  than  those 
in  the  eastern  part  of  the  United  States,  results  of  great  value 
can  undoubtedly  be  obtained  in  many  classes  of  work,  such 
as  the  photography  of  stellar  spectra,  the  measurement  of  the 
heat  radiation  of  the  stars,  etc.  The  immense  amount  of 
light  which  this  mirror  will  collect  should  render  it  particu- 
larly suitable  for  spectroscopic  work  of  all  kinds. 

It  need  hardly  be  said  that  the  100-inch  mirror,  when 
suitably  mounted,  will  play  a  most  important  part  in  the 
scheme  of  research  of  the  Solar  Observatory.  The  investiga- 
tion of  stellar  evolution  frequently  calls  for  adequate  spectro- 
scopic study  of  stars  beyond  the  'reach  of  existing  instru- 
ments. With  the  40-inch  Yerkes  telescope,  for  example,  it 
was  impossible  to  obtain  satisfactory  evidence,  positive  or 
negative,  as  to  the  transition  from  solar  stars  to  those  of  the 
fourth  type.  The  large  number  of  stars  within  the  reach  of 
a  100-inch  reflector  (which  will  give  images  about  ten  times 
as  bright  as  the  40-inch)  should  greatly  increase^ the  chances 
of  finding  possible  intermediate  types,  so  important  in  their 
bearing  upon  the  relationship  of  solar  and  red  stars.  This  is 
only  a  single  instance,  but  it  forcibly  suggests  itself  when  con- 
sidering our  programme  of  research.  In  other  fields  the  large 
reflector  should  be  equally  valuable,  especially  for  the  pho- 
tography of  the  numerous  small  spiral  nebulae,  the  details  of 
which  should  be  brought  out  to  good  advantage  with  a  focal 
length  of  50  feet;  minute  investigation  of  the  larger  nebulae, 
in  the  hope  of  detecting  changes  in  their  form ;  the  study, 
with  very  high  dispersion,  of  the  spectra  of  bright  stars,  etc. 
The  remarkable  calm  of  the  summer  nights  on  Mount  Wilson 
should  assist  materially  in  all  of  this  work,  since  vibration  of 
the  tube,  caused  by  the  wind,  would  undoubtedly  be  a 
serious  drawback  under  less  favorable  conditions. 


242  STELLAR  EVOLUTION 

It  is  impossible  to  predict  the  dimensions  that  reflectors 
will  ultimately  attain.  Atmospheric  disturbances,  rather 
than  mechanical  or  optical  difficulties,  seem  most  likely  to 
stand  in  the  way.  But  perhaps  even  these,  by  some  process 
now  unknown,  may  at  last  be  swept  aside.  If  so,  the  astron- 
omer will  secure  results  far  surpassing  his  present  expecta- 
tions. 


CHAPTER  XXV 
OPPORTUNITIES  FOR  AMATEUR  OBSERVERS 

I  SHALL  never  forget  my  delight,  when  as  a  boy,  I  first 
learned  of  the  spectroscope.  Its  extraordinary  achievements, 
and  the  endless  possibilities,  vaguely  imagined,  of  its  further 
applications  in  astronomical  research,  filled  me  with  enthusi- 
asm, and  kindled  a  strong  desire  for  immediate  work.  The 
visual  study  of  flames,  with  a  simple  one-prism  spectroscope, 
aroused  an  ambition  to  photograph  spectra.  This  was  soon 
accomplished,  by  substituting  an  ordinary  camera  for  the 
observing  telescope.  But  the  scale  of  the  photographs  was 
too  small,  so  I  built  a  longer  camera  of  wood.  Later,  when 
Rowland  was  making  his  earliest  gratings,  one  of  the  small- 
est size  was  secured,  and  substituted  for  the  prism.  The 
marvelous  increase  in  resolving  power,  and  the  greatly  aug- 
mented beauty  of  the  solar  spectrum,  led  to  observations  of 
the  solar  prominences,  and  subsequently  to  more  serious 
research.  But  none  of  the  pleasures  of  later  years,  during 
which  I  have  enjoyed  the  privilege  of  using  larger  and  more 
powerful  instruments,  has  surpassed  the  delight  of  the  initial 
work,  much  of  which  was  done  with  simple  and  inexpensive 
apparatus  of  my  own  construction. 

These  remarks  are  called  forth  by  certain  criticisms  I  have 
heard  of  great  modern  observatories.  Some  amateurs,  I 
am  told,  believe  that  their  efforts  are  rendered  futile  by 
the  more  powerful  equipment  and  better  atmospheric  advan- 
tages of  other  investigators.  If  this  feeling  were  well- 
grounded,  it  might  fairly  be  asked  whether  the  great  observa- 
tories are  worth  their  cost.  For  the  history  of  astronomy 
teaches  that  much  of  the  pioneer  work  has  been  done  by 

243 


244  STELLAK  EVOLUTION 

amateurs,  usually  with  modest  means  and  in  unfavorable  cli- 
mates. To  discourage  this  class  of  workers,  unfettered  as 
they  are  by  the  traditions  of  institutions,  and  driven  by  their 
own  initiative  into  unexplored  fields,  would  be  a  serious  error, 
hardly  to  be  atoned  for  by  any  services  the  larger  observa- 
tories can  render. 

We  may  therefore  inquire  whether  useful  work,  of  such 
a  nature  as  to  contribute  in  important  degree  to  the  progress 
of  science,  can  still  be  done  with  simple  and  inexpensive 
instruments.  This  question  may  at  once  be  answered  in  the 
affirmative.  The  results  of  amateur  observations  may  not 
only  be  useful — they  may  equal,  or  even  surpass,  the  best 
products  of  the  largest  institutions.  Great  care  must  be  exer- 
cised in  choosing  the  subject  of  research,  in  constructing  the 
instruments,  in  making  the  observations  by  the  best  methods 
and  at  the  most  favorable  hours,  and  in  the  reduction  and 
discussion  of  the  results.  If  such  precautions  are  observed, 
discouragement  will  soon  give  way  to  confidence  and  success. 

Take,  for  example,  the  direct  photography  of  the  Sun.  A 
2-inch  objective,  of  40-feet  focal  length,  will  give  beautiful 
solar  photographs,  over  4  inches  in  diameter,  perfectly 
adapted  for  the  study  of  the  solar  rotation,  the  proper  motions 
of  the  spots,  and  other  important  purposes.  Details  sepa- 
rated by  less  than  two  seconds  of  arc  will  not  be  resolved  on 
these  photographs,  but  in  many  classes  of  work  little  gain 
would  result  from  increased  resolving  power.  Such  an  ob- 
jective should  be  mounted  so  as  to  send  the  beam  horizontally 
(better  vertically)  across  shaded  ground,  or  within  a  building, 
to  the  photographic  plate.  If  no  coelostat  is  available,  a  small 
mirror,  with  optically  plane  reflecting  surface,  will  serve  the 
needs  of  direct  photography.  It  is  only  necessary  to  mount  it 
on  a  wooden  support,  so  that  it  can  be  held  at  the  angle  required 
to  reflect  sunlight  through  the  objective.  The  exposures— 
made  by  the  rapid  motion  of  a  wooden  shutter,  pierced  by  a 


OPPORTUNITIES  FOB  AMATEUR  OBSERVERS    245 

narrow  slit  with  brass  edges,  mounted  just  in  front  of  the 
plate — are  very  short,  and  the  slight  drift  of  the  solar  image 
during  this  time  can  be  overcome,  when  desired,  by  a  very 
simple  driving  mechanism.  Between  exposures  the  small 
mirror  should  be  shielded  from  the  Sun.  The  apparatus  used 
by  the  American  parties  to  photograph  the  last  transit  of 
Venus  across  the  Sun  was  of  this  type,  except  that  a  4-inch 
objective  and  larger  mirror  were  used. 

It  will  probably  be  found  that  the  best  solar  definition 
occurs  in  the  early  morning,  before  the  ground  is  greatly 
heated.  A  careful  study  should  be  made  of  the  local  con- 
ditions before  selecting  the  hours  of  work. 

Solar  photographs,  made  in  this  way  at  intervals  of  from 
one  to  several  hours,  may  be  combined  in  the  stereoscope 
with  striking  results.  More  important,  however,  would  be 
a  long  series  of  photographs,  made  at  short  intervals,  and 
examined  with  a  kinetoscope.  These  should  show  the  Sun 
rotating  under  one's  eyes,  the  spots  near  the  equator  moving 
more  rapidly  than  those  in  higher  latitudes.  The  effect  of 
proper  motion,  in  causing  some  spots  to  overtake  others  in 
the  same  latitude,  should  also  be  very  finely  brought  out. 
Even  more  interesting,  however,  would  be  the  changing 
forms  of  spots,  and  the  manner  of  their  growth  and  decay, 
which  have  never  yet  been  observed  by  this  method. 

The  same  horizontal  telescope,  with  some  modifications, 
would  give  an  admirable  image  for  spectroscopic  work.  The 
objective  should,  if  possible,  be  of  from  4  to  6  inches  aperture, 
and  from  40  to  60  feet  focal  length.  The  mirror  should  also 
be  increased  in  the  same  ratio,  and  mounted  as  a  coelostat, 
with  its  plane  parallel  to  the  Earth's  axis.  If  the  mirror  is 
very  thick — 3  inches  or  more — its  form  will  be  changed  but 
little  by  sunlight.  A  second  mirror  will  be  needed  to  send 
the  beam  to  the  spectrograph,  as  in  the  Snow  telescope 
(Plate  LVIII).  If  this  arrangement  appears  formidable,  it 


246  STELLAK  EVOLUTION 

should  be  remembered  that  almost  all  the  parts  can  be  made 
of  hard  wood,  thoroughly  soaked  in  melted  paraffine,  to  pre- 
vent warping.  The  bearings  are  practically  the  only  parts 
that  need  be  of  metal.  A  cheap  clock  movement,  with  heavy 
spring,  will  serve  for  a  driving-clock,  or  a  small  electric 
motor  may  be  used.  With  moderate  ingenuity,  any  amateur 
accustomed  to  the  use  of  tools  can  build  such  an  instrument 
for  a  very  small  sum. 

The  spectrograph  is  even  more  simple.  It  should  be  of 
the  Littrow  form  (p.  153),  and  the  aperture  of  the  single 
plano-convex  lens  that  serves  for  both  collimator  and  camera 
should  be  from  1  to  1^  inches.  Its  focal  length  will  be 
determined  by  the  diameter  and  focal  length  of  the  objective 
used  to  form  the  solar  image  on  the  slit.  If  these  are  4 
inches  and  60  feet,  respectively,  the  ratio  will  be  1:180. 
Hence  the  focal  length  of  the  spectrograph  lens  should  be  180 
times  its  aperture,  or  from  15  feet  to  22  feet  6  inches.  The 
grating  should  be  a  2-inch  Rowland,  or,  if  this  is  too  expen- 
sive, a  good  replica  by  Ives,  Wallace,  or  Thorpe.  The  repli- 
cas have  the  disadvantage  of  being  made  on  transparent  films, 
for  use  with  transmitted  light ;  but  they  can  perhaps  be  con- 
verted into  reflecting  gratings  by  silvering. 

The  collimator-camera  lens  should  be  mounted  on  a  ver- 
tical wooden  bracket,  arranged  to  slide  3  or  4  inches  for 
focusing.  The  grating  may  also  have  a  wooden  support, 
consisting  of  a  bracket,  which  can  be  tippled  forward  or 
back,  mounted  on  a  circular  wooden  table,  permitting  rota- 
tion about  a  vertical  axis  in  the  plane  of  the  grating.  Such 
rotation  is  necessary  in  order  to  bring  different  spectra  upon 
the  photographic  plate,  or  to  pass  from  one  region  to  another 
in  the  same  spectrum.  The  height  of  the  spectrum  on  the 
plate  can  be  adjusted  by  tipping  the  grating  forward  or  back. 
It  is  also  necessary  to  make  the  lines  of  the  grating  parallel 
to  the  slit;  this  can  easily  be  done  by  hanging  the  bracket 


OPPOETUNITIES  FOR  AMATEUR  OBSERVERS     247 

from  above,  and  defining  its  position  by  two  side  screws, 
passing  through  wooden  blocks  attached  to  the  circular  table. 
Plate  CIII  shows  a  wooden  lens  and  grating  support  in  reg- 
ular use  as  part  of  a  Littrow  spectrograph  of  18  feet  focal 
length  in  the  laboratory  of  the  Solar  Observatory. 

The  extreme  simplicity  of  the  slit  end  of  the  same  instru- 
ment is  illustrated  by  Plate  CIV.  A  short  slit,  with  one  jaw 
movable  by  a  screw,  is  supported  by  a  tube  fitting  tightly  in 
a  hole  bored  through  a  wooden  bracket.  Below  is  the  plate- 
holder,  held  in  a  frame  that  slides  up  and  down,  permitting 
many  narrow  spectra  to  be  photographed  on  the  same  plate. 
In  another  similar  instrument  the  slit  and  plate-holder  sup- 
port stands  on  a  pier,  and  fits  into  a  partition,  so  as  to  exclude 
all  light  from  the  room  except  that  which  enters  through  the 
slit.1  In  this  case  no  tube  is  necessary  between  the  plate  and 
lens.  The  latter  is  mounted,  with  the  grating,  on  a  pier  at  a 
distance  from  the  slit  equal  to  the  focal  length  of  the  lens. 

In  spite  of  the  simplicity  and  cheapness  of  such  a  spectro- 
graph, no  better  instrument  could  be  asked.  Its  one  draw- 
back— the  reflections  of  the  slit  from  the  surfaces  of  the  lens 
— is  easily  removed  by  placing  a  bar  across  the  lens  (as  shown 
in  Plate  CIV) .  Wooden  spectrographs  are  in  constant  use 
at  the  Solar  Observatory,  and  give  results  which  are  very 
satisfactory. 

Any  of  the  solar  spectroscopic  work  described  in  this  book 
can  be  done  with  such  an  instrument.  The  resolving  power, 
even  with  only  an  inch  aperture,  will  be  sufficient  for  the  sepa- 
ration of  very  close  solar  lines.  The  spectra  of  sun-spots,  the 
solar  rotation,  the  remarkable  differences  between  the  spectra 
of  the  center  and  limb  of  the  Sun,  and  many  other  phenom- 
ena can  be  studied  by  its  aid  with  the  greatest  precision  and 
success.  The  exposures,  it  is  true,  must  be  longer  than  with 

i  This  room  is  part  of  a  long  hall,  for  testing  optical  mirrors,  in  the  Pasadena 
shop  of  the  Solar  Observatory.  By  opening  large  light-tight  doors,  the  hall  can  be 
used  for  the  transmission  of  light  in  the  knife-edge  tests. 


248  STELLAR  EVOLUTION 

a  spectrograph  of  larger  aperture,  but  this  is  not  a  serious 
obstacle.  Indeed,  it  may  be  said  that  at  the  present  time 
only  two  or  three  observatories  in  the  world  are  using  equip- 
ment as  powerful  as  this  for  the  classes  of  solar  work  just 
enumerated. 

I  might  go  on  to  describe  a  wooden  spectroheliograph, 
fitted  up  with  spare  lenses  and  prisms,  which  gave  excellent 
results  with  the  Snow  telescope  before  the  5-foot  spectro- 
heliograph was  completed.  Indeed,  the  photographs  were 
quite  equal  to  those  taken  with  the  latter  instrument,  except 
that  they  did  not  include  the  entire  solar  image,  which  is 
unnecessary  for  many  kinds  of  work.  The  small  cpelostat 
telescope  described  above  would  give  as  good  results  as  the 
Snow  telescope  with  such  a  spectroheliograph,  except  that  the 
exposures  would  be  longer.  The  entire  apparatus  is  easily 
within  the  reach  of  any  intelligent  amateur  of  limited  means. 

Those  who  desire  to  undertake  solar  work  would  do  well 
to  procure  the  Transactions  of  the  International  Union  for 
Co-operation  in  Solar  Research.1  The  aim  of  the  Union  is 
to  encourage  co-operation  among  observers,  in  the  various 
fields  where  this  is  desirable.  For  example,  it  is  impossible, 
in  visual  observations  of  sun-spot  spectra,  for  one  person  to 
make  a  thorough  study  of  more  than  a  limited  region.  By 
mutual  agreement,  the  spectrum  is  therefore  divided  up 
among  many  observers,  who  record  their  results  on  a  common 
plan.  Spectroheliographs,  distributed  from  India  across 
Europe  to  California,  are  also  operated  in  harmony,  and  co- 
operation is  practiced  in  other  fields  as  well.  Apart  from 
such  routine,  however,  every  observer  is  encouraged  to  act 
on  his  own  initiative,  for  the  Solar  Union  recognizes  that 
the  greatest  advances  will  come  from  individual  effort,  which 
no  amount  of  co-operation  can  replace. 

i  Vol.  I  was  published  by  the  University  Press,  of  Manchester,  England,  in  1906. 
Vol.  II  will  soon  appear. 


INDEX 


ABBOT:  teets  of  Mount  Wilson  atmos- 
phere, 129;  solar  radiation,  214;  pyrhe- 
liom'  ter,  215. 

^ABSORPTION:  spectrum,  52;  in  solar 
atmosphere,  53, 68  ;  in  hydrogen  flocculi, 
96 ;  in  stellar  atmospheres,  173. 

ADAMS:  metallic  and  spot  spectra,  159; 

titanium  oxide  in  spots,  162;  spectrum 

of  Ari-twus,  168;  spectrum  of  a  Orionix, 

170 ;  Trapezium  stars,  189 ;  "•Orion''1  type 

^    stars,  1<0. 

ALTITUDES  :  advantages  of  high,  111-20. 

AMATEURS:  opportunities  for,  27,  243-49. 

Andromeda  nebula,  41,  44. 

ANOMALOUS  DISPERSION  and  solar  phe- 
nomena, 148. 

Antares,  195. 

Arcturus:  spectrum,  168;  heat  radiation, 

172,  173. 
4*  ASTROPHYSICS  :  relation  to  astronomy  and 

physics,  6.. 

I  ATMOSPHERE:,  absorption  in  Earth's,  63, 
114,  128;  unsteadiness,  111,  112,  127. 

BARNARD:  photography  of  Milky  Way, 
30-33,  128;  micrometric  observations, 
103 ;  comparative  photographs  atMount 
Wilson  and  Lake  Geneva.  128, 129;  tests 
of  Mount  Wilson  definition,  129. 

BARNARD  AND  RITCHEY:  photography 
of  corona,  76. 

Betelgeuze:  spectrum,  170. 
K'  BINARIES  :  spectroscopic,  103. 

BOLOMETER,  21o. 

BOYS:  stellar  hr  at,  171. 

BRUCE  SPECTROGRAPH,  104,  167. 

BRUCE  TELESCOPE,  29-33. 

BURNHAM:  discovi  ries  with  small  tele- 
scope, 27;  observations  with  ^erkes 
refractor,  103. 

CALCIUM  :  linns,  H  and  K,  84,  91 ;  flocculi, 

85-93,  143,  147  ;  vapor,  radial  motion  of, 
*     9'-'. 

CALCIUM  HYDRIDE:  in  sun-spots,  163. 
CALVERT:  corona,  73. 
CAMERA  LENS  :  stellar  photography  with, 

28-32. 

CAMPBELL:  stellar  motions,  lO.'i. 
Canes  Venatici :  spiral  nebula  in,  38,  39. 
CARBON:    in  chromosphere,  80;    in  red 

stars,  195. 
CARNEGIE.  ANDREW:   establishment  of 

Carnegie  Institution,  109. 


CARNEGIE  INSTITUTION:  purpose  of,  109. 

CHAMBERLIN:  criticisms  of  nebular 
hypothesis,  182-86;  planetesimal  hy- 
pothesis, 208-10. 

CHROMOSPHERE,  15,  84;  spectrum,  78; 
"flash"  spectrum,  80. 

COELOSTAT,  75, 109,  245;  advantages,  131; 
Snow  telescope,  133,  134 ;  "  tower  "  tele- 
scope, 231. 

CO-OPERATION  in  research,  98,  218,  249. 

CORNU:  telluric  lines,  63,  64. 

CORONA,  16,  73-75 ;  spectrum  of,  74. 

CROSSLEY  REFLECTOR,  42,  45. 

Cygnus:  nebula  in,  44. 

DARWIN,  CHARLES  :  Origin  of  Species,  1 ; 

correlation  in  research,  97. 
DARWIN,  SIR  GEORGE  :  tid^l  friction,  183; 

meteoroidal  swarm,  183,  204. 
DESLANDRES  :  level  of  calcium  flocculi, 

90;  spectra  of  floccn]i,96;  spectmhelio- 

graph,  96;  Foucault  siderostat,  131. 
DRAPER,  41,  54. 

ECHELON,  65. 

ECLIPSE:  solar,  73;  apparatus,  75. 

ELLERMAN  :  work  with  Kenwood  spectro- 
heliograph,  86;  work  w  th  Rumford 
spectroheliograph,  89;  work  with  5-foot 
spectroheliograph,138;  photography  of 
spot  spectra,  152,  163. 

EVERSHED  :  spectroheliograph,  96. 

EVOLUTION:  early  views,  1;  general 
problem,  3. 

FACULAE,  15,  71,  72,  85,  86,  90, 146. 

FLAGSTAFF.  123. 

"FLASH"  SPECTRUM,  80. 

FLOCCULI:  calcium.  85;  daily  motion,  87, 
142,146;  minute, 89;  levels, 90;  eruptive, 
9^;  hydrogen,  P4;  iron,  96;  h  liographic 
positions,  114;  proper  motions,  146;  level 
of  calcium  and  hydrogen,  147;  levels, 
150 ;  areas,  150,  217. 

FOUCAULT:  siderostat,  131. 

FOWLER:  magnesium  hydride  in  spots, 
163;  titanium  oxide  in  re  1  stars,  195. 

Fox:  measures  of  Kenwood  plates,  144. 

FKAUXHOFER:  dark  lines  in  solar  spec- 
trum, 47;  objective  prism,  189;  t-tellar 

ft  spectra,  189. 

*FROST:    stellar   spe^troscopy,   104,    105; 

heat  radiation  of  sun-spots,  149 ;  Trape- 

6     zium  stai  s,  189 ;  "Orion."  type  stars,  190. 

FURNACE:  electric,  160. 


249 


250 


STELLAR  EVOLUTION 


GALE:  metallic  spectra,  159. 

GALILEO  :  early  discoveries,  9. 

GLOBE  MEASURING  MACHINE,  144. 
ft  GRATINGS:     Rowland,    56-59,    235,    236; 
Michelson,  65,  66,  236;  Jewell,  66. 

GREENWICH  OBSERVATORY:  spot  posi- 
tions, 144. 

HARVARD  OBSERVATORY:  refractor,  41; 
objective  prism,  189;  stellar  spectra, 
201. 

HELIOMICROMETER,  144. 

HELIUM:  in  Sun,  78,  79;  terrestrial,  78; 
in  "Orion"  stars,  79;  in  nebulae,  190. 

HERSCHEL,  SIR  JOHN:  clusters  and 
nebulae,  46. 

HERSCHEL,  SIR  WILLIAM:  clusters  and 
nebulae,  46;  condensation  of  nebulae, 
187. 

HIGGS:  map  of  solar  spectrum,  62. 

HOOKER,  J.  D. :  gift  of  100-inch  mirror, 
238. 

HOOKER  EXPEDITION,  30, 128. 

HOOKER  TELESCOPE,  238-42. 
|    HUGGINS:    stellar   spectra,   53;    promi- 
nences, 54,  76;  spectrum  of  nebulae,  54; 
1     helium,  79;  stellar  motions,  105 ;  stellar 
;     heat,  171;  stellar  evolution,  199;   tem- 
perature of  nebulae,  207. 
h HYDROGEN:    spectrum,  78;  in  stars,  79, 
170,  191,  193,  195,  ll<9,  200,  206 ;  in  promi- 
nences, 83;  in  mbulae,  190;  in  meteor- 
ites, 20>. 

HYDROGEN  FLOCCULI,  93-95;  level,  com- 
pared with  calcium,  147. 

j*  INTERFEROMETER,  65. 

JANSSEN:     prominences,    54,    76;     solar 

photography,  70. 

t\  JEWELL:  telluric  lines,  63;  gratings,  66. 
f  JULIUS  :    anomalous    dispersion  theory, 

148. 

KAPTEYN  :  structure  of  universe,  202. 

KEELER:  spiral  nebulae,  3,  45;  pho- 
tography with  Crossley  reflector.  42; 
Sat  urn's  rings,  182;  chief  nebular  line, 
205. 

KENWOOD  OBSERVATORY,  83;  spectro- 
heliograpli,  84;  spot  spectra,  152. 

KIRCHHOFF:  explanation  of  solar  spec- 
trum, 51-53. 

KIRKWOOD  :  nebular  hypothesis,  185. 

KODAIKANAL    OBSERVATORY,   119. 

LABORATORY:  Yerkes  Observatory,  107; 
Solar  Observatory,  l-"»6. 

LANE'S  LAW,  191. 

LANGLEY:  sun-spots,  69;  photospheric 
granules,  69-71;  color  of  Sun,  193;  solar 
radiation,  214;  bolometer,  215. 


LAPLACE:  nebular  hypothesis,  2,  175-86. 

LICK  OBSERVATORY,  42, 119,  120,  205. 

LICK  TELESCOPE,  26,  41. 

LITTROW  SPECTROGRAPH;  laboratory, 
156;  of  Snow  telescope,  134,  153;  of 
"tower"  telescope,  232;  wooden,  246-48. 

LOCKYER:  prominences,  54,  76;  helium, 
78;  sun-spot  spectra,  151;  dissociation 
in  sun  spots,  151 ;  temperature  of  sun- 
spots,  152;  temperature  of  stars,  173; 
enhanced  lines,  194;  stellar  classifica- 
tion, 194,  207;  meteoritic  hypothesis, 
204-8. 

MAGNESIUM  HYDRIDE:  in  sun  spots,  163. 

MAGNIFYING  POWER,  22. 

Mars:  period  of  inner  satellite,  183. 

MAUNDER:  band  lines  in  spots,  152. 

MAXWELL  :  Saturn's  rings,  182. 

METEORITES  :  spectra,  205. 
/MICHELSON:   interferometer,  65;  stand- 
'\     ard    wave-lengths,    65,    echelon,     65; 
gratings,  65,  66,  236. 

MILKY  WAY  :  photographs  of,  30-33. 

MILLS  SPECTROGRAPH,  167. 

MIRROR  :  60-inch,  figuring,  219-26 ;  method 
of  testing,  222,  224-26;  100-inch,  238-41. 

MIRRORS  :  distortion,  137, 138,  231-35,  240. 

MOMENTUM  :  moment  of,  185. 

MOON  :  photography  of,  33. 

MONT  BLANC,  119. 

MOULTON:  criticisms  of  nebular  hypoth- 
esis, 182-86. 

MOULTON  AND  CHAMBERLIN:  planetesi 
mal  hypothesis,  208-10. 

MOUNT  ETNA:  expedition  to,  116-19. 

MOUNT  HAMILTON,  119, 120. 

MOUNT  WILSON,  123-30. 

MOUNT  WILSON  SOLAR  OBSERVATORY: 
origin,  110;  plan  of  research,  121 ;  site, 
123-30;  Snow  telescope,  132-38;  work 
with  spectroheliograph,  139-50;  sun- 

f  spot  spectra,  153-64;  laboratory,  155-58, 

v,  160;  stellar spectroscopy,  167-71;  60-inch 

]  reflector,  219-29;  "tower"  telescop", 
232-35;  100-inch  reflector,  238-42. 

MOUNTAINS  :  as  observatory  sites,  113-30. 


NEBULA:  spiral  in  Canes  Venatici, 
in  Andromeda,  41,  44,45;  in  Ci/gims,  44; 
in  Orion,  44,  188-90;  in  Draco,  54;  La- 
place's, 177. 

'  NEBULAE:  spiral,  7,  38,  39,  41,  44,  45,  1*8, 
203;  relationship  to  stars,  3,  32,  55,  177, 
187-90,  198-201,  206,  207,  209,  210;  and 
clusters,  18,  46,  47,  54;  in  Milky  Way,  31, 
32;  in  Pleiades,  44,  129,  198,  202;  spec- 
trum of,  54,  190,  203;  condensation, 
17,fe-81,  183,  187,  200,201,212;  planetary, 
187;  temperature  of,  207. 
NEBULAR  HYPOTHESIS,  2,  175-86. 

tNEBULUM,  204. 

Neptune:  orbits  of  satellites,  183. 


INDEX 


251 


'  NEWTON  :  analysis  of  sunlight,  47 ;  advan- 
tages of  high  altitudes,  111. 
NICHOLS  :  stellar  heat,  172. 

OLMSTED  :  calcium  hydride  in  spots,  163. 

Origin  of  Species,  1. 

Orion   NEBULA,    44,    188-90;     Trapezium 

stars,  188, 190. 
"Orion"  TYPE  STABS,  79, 190. 

PHOTOGRAPHY:  advantages,  28;  star 
trails,  29;  with  camera,  29-33;  clusters, 
33,  44;  Moon,  33,  34;  with  Yerkes  tel- 
escope, 33-36 ;  with  reflectors,  40-45 ;  of 
nebulae,  41-45,  203;  solar  spectrum,  60- 
64,  247,  248 ;  Sun,  70-72,  244,  245 ;  eclipse, 
74-76;  with  spectroheliograph,  81-96, 
139-42,  149,  150,  236,  237,  248;  stellar 
spectra,  104, 105, 165-71, 189-97, 203 ;  coro- 
na, without  eclipse,  115-18;  Milky  Way, 
128, 129;  with  Snow  telescope,  137, 138; 
sun-spot  spectra,  152-54 ;  metallic  spec- 
tra, 158-62. 

PHOTOSPHERE  :  structure,  69-72. 

PHYSICS:  fundamental  importance  of,  5. 

Pic  DU  MIDI,  119. 

PICKERING,  E.  C.:  objective  prism,  189; 
stellar  spectra,  201. 

PIKE'S  PEAK:  expedition  to,  115, 116. 

PLANETESIMAL  HYPOTHESIS,  208-10. 

Pleiades,  18,  44, 129, 177, 198,  202. 

POTSDAM  ASTROPHYSICAL  OBSERVATORY, 

100,  167. 

S  PRISM:  formation  of  spectrum,  48;  ob- 
jective, Frauntiofer,  189;  objective, 
Pickering,  189;  glass,  236. 

PROMINENCES,  15;  nature  of,  76;  without 
eclipse,  77;  spectrum,  78;  seen  with 
open  slit,  81;  quiet-cent  and  eruptive, 
81,  93;  photography  of,  82;  H  and  K 
in,  84. 

PYRHELIOMETER,  215. 

QUARTZ:  fused,  for  telescope  mirrors, 
233-35. 

RADIOMETER,  172. 

RADIUM,  212,  213. 

RAMSAY  :  discovery  of  helium,  78. 

REVERSING  LAYER,  143. 

RITCHEY:  photograp1  s  of  Moon,  33,  34; 
photography  with  Yerkes  telescope,  33, 
36;  24-inch  reflector,  43;  photography 
with  reflector,  44;  telescope  construc- 
tion, 43,  ^19-30;  100-inch  reflector,  238-40. 

RITCHEY  AND  BARNARD:  photography 
of  corona,  76. 

ROBERTS  :  Andromeda  nebula,  41. 

ROSSE:  6-foot  reflector,  38,  39. 

ROTATION  :  solar,  by  spots,  143;  by  facu- 
lae,  143;  by  flocculi,  143,  146. 

ROWLAND:  gratings,  56-59 ;  composition 
of  Sun,  62 ;  map  of  solar  spectrum,  62; 
solar  spectrum  wave-lengths,  62. 


RUMFORD  SPECTROHELIOGRAPH,  88. 

RUNGE:  helium,  79. 

RUTHERFURD:  stellar  spectra,  2,  53; 
gratings,  57. 

SATURN:  ring  system,  179;  constitution 
of  rings,  182;  revolution  of  rings,  184. 

SCHUSTER  :  stellar  evolution,  199. 
f  SECCHI:  st'-llar  spectra,  53;  prominences, 
76 ;  classification  of  stellar  spectra,  170. 

Sirius:  spectrum,  191. 

SMITH,  PIAZZI:  Teneriffe  expedition,  119. 

SMITHSONIAN  OBSERVATORY,  214. 

SNOW  TELESCOPE,  132-38. 

SOLAR  UNION,  218,  248. 

SPECTRA:  stellar,  2,  18,  104,  105,  164-71, 
189-91, 193-203,  207,  208;  nebulae,  18,  54, 
203-7;  solar,  47,  51-53,  60-64,  215;  con- 
tinuous, 49,  51 ;  bright-line,  49-53.  60,  61, 
107, 141, 157-63,  L05;  dark-line,  51-54,  93, 
94;  prominences,  54,  76-81 ;  grating,  58- 
60;  chromosphere,  78-81;  "flash,"  80; 
flocculi,  85,  86,  90,  91,  93-96 ;  faculae,  90 ; 
sun-spots,  108,  151-64 ;  arc,  159-62;  elec- 
tric furnace,  160, 161 ;  Saturn's  ring,  182 ; 
Sun,  center  and  limb,  192;  "enhanced" 
lines,  194. 

SPECTROGRAPH  :  Bruce,  104, 167 ;  Littrow, 
134,  153;  laboratory,  156;  Mills,  167; 
Potsdam,  167;  grating,  for  stars,  167, 
228;  of  "tower "tele scope,  232;  wooden, 
246-48. 

SPECTROHELIOGRAPH  :   principle  of,  82; 
Kenwood,  84 ;  Rumford,  88 ;  use  of  dark 
lines,  93;    5-foot,   139;    operation,  141; 
30-foot,  233;  future  development,  236. 
••'  SPECTROSCOPE:    Kirchhoff's,    51;    plane 
grating,  58,  77;  concave  grating,  59,  60; 
objective  prism,  80,  189. 
'   SPECTROSCOPIC  BINARIES,  105. 

SPENCER  :  nebulae,  47. 

SPURIOUS  DISK,  23. 

';  STARS:  clusters,  18,  46 ;  colors,  18, 170, 173, 
191,  193,  195,  199;  size  of  image,  23; 
clouds,  31 ;  spectra,  53,  104,  105,  165-71, 
173,  174,  189-203,  207,  208,  241 ;  twinkling, 
111 ;  heat  radiation,  171 ;  red,  173,  195- 
97,208;  temperature.  173;  helium,  190; 
"•Orion"1  type,  190;  white,  191 ;  dark,  197. 

STEREOCOMPARATOR,  147. 

SUN:  visual  appearance,  15,68;  composi- 
tion, 16;  activity,  16,  217;  as  a  star,  17; 
line  absorption,  53;  absorption  in  at- 
mosphere, 68;  direct  photography,  70, 
245;  inclination  of  axis,  143;  rotation, 
143,  146;  contraction,  191;  spectra  of 
center  and  limb,  192;  radiation,  212-16. 

SUN-SPOTS,  15,  69;  periodicity,  16;  level, 
71,148;  heliograpiiic  positions,  143-16; 
dissociation  in,  151;  darkness,  151,  163; 
spectrum,  151-64;  temperature,  163. 

TELESCOPE:  magnifying  power,  22; 
brightness  of  image,  22;  resolving 
power,  23 ;  large  and  small,  24-27 ;  fixed, 
131,232;  "tower,"  232. 


252 


STELLAR  EVOLUTION 


TELESCOPE  :  reflecting,  Rosse,  38 ;  devel- 
opment of,  38-45 ;  advantages,  42 ;  Cross- 
ley,  42,  45;  24-inch,  43;  Snow,  132-38,- 
60-inch,  219-30;  100-inch,  238-42. 

TELESCOPE:  refracting,  21,  230;  Yerkes, 
25,  26,  33-36,  43,  88,  101-4;  Lick,  26,  41; 
Burnham's  27;  camera,  28-M3;  Bruce, 
29,  30;  Kenwood,  33,  84;  development 
of,  41. 
I  TELLURIC  LINES,  63,  64. 

TENERIFFE  EXPEDITION,  119. 

TITANIUM  OXIDE  :  in  sun-spots,  162. 

"TOWER"  TELESCOPE,  232. 

Trapezium  STARS,  188, 190. 

TURNER:  coelostat,  132. 
«  TWINKLING  OF  STARS,  111. 

Uranus:  orbits  of  satellites,  183. 


Vega:  heat  radiation,  172,  173. 
|  VOGEL  :  stellar  motions,  105. 

WADSWORTH  :  reflector  mounting,  43. 
WILSON:  sun-spots  as  cavities,  71. 

YERKES  OBSERVATORY  :  policy,  98 ;  origin, 
99;  plan  of  building,  100;  instrument 
and  optical  shops,  106;  spectroscopic 
laboratory,  107;  site,  113;  coelostat 
room,  172. 

YERKES  REFRACTOR  :  photography,  33-36, 
88;  mounting,  34,  101;  compared  with 
reflector,  44;  objective,  101;  operation 
of,  102. 

YouivG:  discovery  of  "flash"  spectrum, 
80 ;  prominences,  81 ;  H  and  K  in  prom- 
inences, 84;  calcium  flocculi,  85;  pho- 
tography of  spot  spectra,  152. 


PLATE  II 


DIRECT  PHOTOGRAPH,  SHOWING  THE  SUN  AS  IT  APPEARS  TO  THE  EYE 


PLATE  III 


THE  SOLAK  CHKOMOSPHEKE  AND  PHOMINENCES 


PLATE  IV 


____ 


•••••BBHBBB III  I  III  i    |  :  HI 


FIG.  1 

CHAEACTEEISTIC  SPECTRA  OF  (a)  WHITE,  (6)  YELLOW,  AND  (c)  RED  STARS 

(Huggins) 


FIG.  2 

THE  SOLAR  CORONA 
Photographed  by  Yerkes  Observatory  Eclipse  Expedition,  May  28, 1900  (Barnard  and  Ritchey) 


PLATE  VI 


STAR  TRAILS  PHOTOGRAPHED  WITH  2)^-iNCH  PORTRAIT  LENS 
(Ritchey) 


PLATE  VII 


THE  BRUCE  TELESCOPE  OF  THE  YERKES  OBSERVATORY 


PLATE  VIII 


STAR  CLUSTER  Messier  11  AND  THE  SURROUNDING  ^MILKY  WAY 
Small-scale  photograph  taken  with  lantern  lens  (Barnard) 


PLATE  IX 


STAR  CLUSTER  Messier  11  AND  THE  SURROUNDING  MILKY  WAY 
Larger-scale  photograph  taken  with  10-inch  Bruce  telescope  (Barnard) 


PLATE  X 


THE  MILKY  WAY  NEAR  p  Ophiuchi 
Photographed  with  10-inch  Bruce  telescope  (Barnard) 


PLATE  XI 


STAB  CLUSTEB  Messier  11 
Large-scale  photograph  taken  with  40-iach  Yerkes  telescope  (Ritchey) 


PLATE  XII 


THE  MOON 
Photographed  with  the  12-inch  Kenwood  refractor  (Ritchey) 


PLATE  XIII 


LUNAR  CRATER  Theophilus  AND  SURROUNDING  REGION 
Photographed  with  the  40-inch  Yerkes  refractor  (Ritchey) 


PLATE  XIV 


THE  40-iNCH  REFRACTOR  OF  THE  YERKES  OBSERVATORY 


PLATE  XVI 


90-FOOT  DOME  OF  THE  YEBKES  OBSERVATORY 


PLATE  XVII 


EYE-END  OF  YERKES  TELESCOPE 
Showing  double-slide  plate-holder 


PLATE  XVIII 


THE  24-iNCH  REFLECTOR  or  THE  YERKES  OBSERVATORY 


PLATE  XIX 


STAR  CLUSTER  Messier  13 
Photographed  with  the  24-inch  reflector  of  the  Yerkes  Observatory  (Ritchey) 


PLATE  XX 


STAB  CLUSTEK  Messier  13 
Photographed  with  the  40-inch  Yerkes  refractor  (Ritchey) 


PLATE  XXI 


THE  GREAT  NEBULA  IN  Orion 
Photographed  with  the  24-inch  reflector  (Kitchey) 


PLATE  XXIII 


SIR  WILLIAM  HUGGINS 


X 


PH 


M  OIST   AIR      DRY    AIR 


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MOIST  AJR:     DRY  A  I  R 


f) 


PLATE  XXVI 


LANGLEY'S  DBAWING  OF  THE  TYPICAL  SUN-SPOT  OF  DECEMBEB,  1873 


§  <•? 


OB  s 


i  -s 

o  § 

H  <» 

B  s 

w 

s 

i 


P-i      ',3 

CC       o 

o 

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PLATE  XXXII 


(a) 


(6) 


(c) 


. 


BRIGHT  H  AND  K  LINES  ON  THE  DISK  (a,  &,  and  c),  IN  THE 
CHROMOSPHERE  (6)  AND  IN  A  PROMINENCE  (a) 


PLATE  XXXIV 


SPECTKOHELIOGKAPH  ATTACHED  TO  12-INCH  KENWOOD  REFRACTOB 


PLATE  XXXV 


ERUPTIVE  PROMINENCE  PHOTOGRAPHED  WITH  THE  KENWOOD  SPECTROHELIOGRAPH 
March  25, 1895,  10h  40m.    Height  of  prominency  162,000  miles 


PLATE  XXXVI 


ERUPTIVE  PROMINENCE  SHOWING  PLATE  XXXV  PHOTOGRAPHED  18  MINUTES  LATER 
Height  of  prominence,  281,000  miles 


PLATE  XXXVII 


RUMFOKD  SPECTROHELIOGRAPH  ATTACHED  TO  40-INCH  YERKES  REFRACTOR 


PLATE  XXXVIII 


THE  SUN,  SHOWING  THE  CALCIUM  FLOCCULI 
August  12,  1903,  8fc  52m 


a  s 

85 

2  § 

PR    d 


II 


B    5 

u     o> 

U 

M  ^ 
«    § 

B    CO 


B    S 


PLATE  XL 


FIG.  1.—  3h  40m.    Second  slit  set  on 


FIG.  2—  3&  31m.     Second  slit  set  on  H2.    Same  re  gion  of  the  Sun 
as  that  shown  in  Fig.  1 

MINUTE  STRUCTURE  OF  THE  CALCIUM  FLOCCULI 


PLATE  XLI 


I 


FIG.  1 
PKISM  TRAIN  OF  THE  RUMFORD  SPECTROHELIOGRAPH 


FIG.  2 


* 


o   o 


3  JS 

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o 

-a 


I- 

' 


PLATE  XLIII 

N 


FIG.  1.— 3h  57m.    Calcium  flocculi  (K9) 


W 


FIG.  2.— llh  Om.    Hydrogen  flocculi  (Hy)  (bright  eruptive  flocculi  west  of  s-pot) 
HYDROGEN  AND  CALCIUM  FLOCCULI,  JULY  7,  1903 


i 


PLATE  XLVI 


THE  BKUCE  SPECTKOGKAPH  OF  THE  YERKES  OBSERVATORY 
Mounted  on  its  carriage,  with  constant  temperature  case  removed 


>      s- 

XI 


~      PQ 


H 

a  _ 

H     1» 


O         H 

B   i 


i 


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I 


PLATE  LVI 


FIG.  1.— At  the  Yeik  s  Observatory.    Exposure  4'jm 


FIG.  '^. — At  Mount  Wilson.    Exposure  41m 

STAB  CLUSTER  Messier  35 
Photographed  with  the  Bruce  telescope  (Barnard) 


3 


PLATE  LXIII 


DIEECT  PHOTOGRAPH  or  THE  SUN 
August  25, 1905,  6h  09m  A.  M. 


PLATE  LXIV 


THE  SUN.  PHOTOGRAPHED  WITH  THE  S-FOOT  SPECTROHELIOGRAPH 

August  25, 1£06,  6h  22m  A.  M. 
Camera  slit  set  on  Hi  line  of  calcium 


PLATE  LXV 


THE  SUN,  PHOTOGRAPHED  WITH  THE  S-FOOT  SPECTROHELJOGBAPH 

August  25, 1906,  6t  18m  A.  M. 
Camera  slit  set  on  EL  line  of  calcium 


PLATE  LXVI 


THE  SUN,  PHOTOGRAPHED  WITH  THE  S-FOOT  SPECTROHELIOGBAPH 

August  25, 1906,  6^  36m  A.  M. 
Camera  slit  set  on  H$  line  of  hydrogen 


PLATE  LXVII 


THE  SUN,  PHOTOGRAPHED  WITH  THE  S-FOOT  SPECTROHELIOGRAPH 

August  25,  1/06,  6h  2Sm  A.  M. 
Camera  slit  set  on  the  iron  line  A  4046 


PLATE  LXVIII 


FIG.  1.— 7h  46m.    Hydrogen  Flocculi  FIG.  2.— 7h  54m.    Iron  Flocculi 

(HS)  (A  4046) 

HYDROGEN  AND  IRON  FLOCCULI  PHOTOGRAPHED  WITH  THE  S-FOOT  SPECTROHELIOGRAPH, 

NOVEMBER  13,  1907 


PLATE  LXX 


THE  HELIOMICROMETER 


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p    s 

§  £ 

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-2 


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H     a 

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£  £ 

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H 


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PLATE  LXXXIII 


53CO 


5400 


Carbon 
Arc 


132  Scl> j. 

(IV) 


a  Orionh 
(HI) 


FIG.  1 

REGION  OF  YELLOW  CAEBON  FLUTING  IN  ELECTBIC  ABC.  FOURTH 

TYPE  STAB  (132  Schjellerup),  AND  THIRD 

TYPE  STAB  (a  Orionis} 


FIG.  2 

SPECTBA  OF  FOUB  FOUBTH  TYPE  STABS 

Photographed  with  the  40-inch  Yerkes  refractor,  showing  how  the  dark  carbon 
band  becomes  stronger  as  the  star  cools 


PLATE  LXXXVI 


THE  PLEIADES 
Photographed  with  the  24-inch  reflector  of  the  Ycrkes  Observatory  (Ritchey) 


PLATE  LXXXVII 


NEBULA  IN  Cygnus,  N.  G.  C.  6992 
Photographed  with  the  24-inch  reflector  (Ritchey) 


PLATE  LXXXVIII 


SPIKAL  NEBULA  Messier  51  Canum  Venaticorum 
Photographed  with  the  24-inch  reflector  (Ritchey) 


PLATE  LXXXIX 


SPIRAL  NEBULA  Messier  101 
Photographed  with  the  24-inch  reflector  (Ritchey) 


PLATE  XC 


SPIRAL  NEBULA  Messier  33  Trianguli 
Photographed  with  the  24-inch  reflector  (Ritchey) 


PLATE  XCIV 


60-INCH    DISK   AFTER    BOTH    SURFACES    HAD    BEEN    FlNE-GROUND    AND    POLISHED 


o 


M 

W 
H 
<3 
^ 
PM 


PLATE  XCVII 


FIG.  1 


FIG.  2 


FIG.  3  FIG.  4 

VARIOUS  MIRROR  COMBINATIONS  IN  60-iNCH  REFLECTING  TELESCOPE 


PLATE  XCVIII 


MOUNTING  OF  60-iNCH  REFLECTING  TELESCOPE 
Under  construction  in  Pasadena  instrument  shop  of  the  Solar  Observatory 


D 

W 
H 
«4 
^ 
PH 


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